Youtube apiv3 に詳しい方

Question

UserがWeb上から動画をアップロードさせる仕組みを作りたいです。

https://developers.google.com/youtube/v3/code_samples/php?hl=ja#resumable_uploads

公式のサンプルコードだとOAuth認証しないと、アップロードできません。Userがgoogleアカウントを持っていなくてもアップロードできるように認証を挟まずにアップロードさせたいのです。

可能なのでしょうか？

GoogleDevelopperConsole->OAuth->サービスアカウント からサービスアカウント取って、試してみましたが、
An client error occurred: Failed to start the resumable upload (HTTP 401: youtube.header, Unauthorized)

と返ってきます。考えられる原因は何でしょうか？
（一応、コード載せますm(_ _)m）Cakephpでの記述です。

public function resumable_upload(){
App::import('Vendor', 'google-api-php-client-master/src/Google/Client');
App::import('Vendor', 'google-api-php-client-master/src/Google/Service/YouTube');
$htmlBody ='';

	// サービスアカウント名（メールアドレス）
	$service_account_name ='service_accont_name';
	// P12キーファイルのパス
	$key_file_location = 'path/to/key';
	session_start();

	if ( !strlen($service_account_name)
   		|| !strlen($key_file_location)) {
		echo missingServiceAccountDetailsWarning();
	}

	$client = new Google_Client();
	$client->setScopes('https://www.googleapis.com/auth/youtube');
	//tokenをセット
	if (isset($_SESSION['service_token'])) {
		$client->setAccessToken($_SESSION['service_token']);
	}
	//Credentials(証明書)用の設定
	$key = file_get_contents($key_file_location);
	$cred = new Google_Auth_AssertionCredentials(
		$service_account_name,
		array('https://www.googleapis.com/auth/youtube.upload'),//$scope
		$key
	);
	
	$client->setAssertionCredentials($cred);
	//トークンのリフレッシュ
	if($client->getAuth()->isAccessTokenExpired()) {
		$client->getAuth()->refreshTokenWithAssertion($cred);
	}
	$_SESSION['service_token'] = $client->getAccessToken();

	//apiを操作用？インスタンスを生成
	$youtube = new Google_Service_YouTube($client);

	if (isset($_SESSION['token'])) {
	  $client->setAccessToken($_SESSION['token']);
	}

);

	// Check to ensure that the access token was successfully acquired.
	if ($client->getAccessToken()) {
		echo 'getAccessToken true <br>';
	  try{
		    // REPLACE this value with the path to the file you are uploading.
		    $videoPath = 'videopath';

		    // Create a snippet with title, description, tags and category ID
		    // Create an asset resource and set its snippet metadata and type.
		    // This example sets the video's title, description, keyword tags, and
		    // video category.
		    $snippet = new Google_Service_YouTube_VideoSnippet();
		    $snippet->setTitle("Test title");
		    $snippet->setDescription("Test description");
		    $snippet->setTags(array("tag1", "tag2"));



		    // Numeric video category. See
		    // https://developers.google.com/youtube/v3/docs/videoCategories/list 
		    $snippet->setCategoryId("22");

		    // Set the video's status to "public". Valid statuses are "public",
		    // "private" and "unlisted".
		    $status = new Google_Service_YouTube_VideoStatus();

		    $status->privacyStatus = "public";

		    // Associate the snippet and status objects with a new video resource.
		    $video = new Google_Service_YouTube_Video();
		    $video->setSnippet($snippet);
		    $video->setStatus($status);

		    // Specify the size of each chunk of data, in bytes. Set a higher value for
		    // reliable connection as fewer chunks lead to faster uploads. Set a lower
		    // value for better recovery on less reliable connections.
		    $chunkSizeBytes = 1 * 1024 * 1024;

		    // Setting the defer flag to true tells the client to return a request which can be called
		    // with ->execute(); instead of making the API call immediately.
		    $client->setDefer(true);

		    // Create a request for the API's videos.insert method to create and upload the video.
		    $insertRequest = $youtube->videos->insert("status,snippet", $video);
		    // Create a MediaFileUpload object for resumable uploads.
		    $media = new Google_Http_MediaFileUpload(
		        $client,
		        $insertRequest,
		        'video/*',
		        null,
		        true,
		        $chunkSizeBytes
		    );
		    $media->setFileSize(filesize($videoPath));


		    // Read the media file and upload it chunk by chunk.
		    $status = false;
		    $handle = fopen($videoPath, "rb");
		    debug($handle);
		    while (!$status && !feof($handle)) {
		      $chunk = fread($handle, $chunkSizeBytes);
		      $status = $media->nextChunk($chunk);
		    }

		    fclose($handle);

		    // If you want to make other calls after the file upload, set setDefer back to false
		    $client->setDefer(false);

		    //text 成形

		    $htmlBody .= "<h3>Video Uploaded</h3><ul>";
		    $htmlBody .= sprintf('<li>%s (%s)</li>',
		        $status['snippet']['title'],
		        $status['id']);
	  } catch (Google_ServiceException $e) {
	    $htmlBody .= sprintf('<p>A service error occurred: <code>%s</code></p>',
	        htmlspecialchars($e->getMessage()));
	  } catch (Google_Exception $e) {
	    $htmlBody .= sprintf('<p>An client error occurred: <code>%s</code></p>',
	        htmlspecialchars($e->getMessage()));
	    echo $htmlBody; ・・・☆ここで401エラーが吐かれます↓

An client error occurred: Failed to start the resumable upload (HTTP 401: youtube.header, Unauthorized)
}

	  $_SESSION['token'] = $client->getAccessToken();
	} else {
	  // If the user hasn't authorized the app, initiate the OAuth flow
	  $state = mt_rand();
	  $client->setState($state);
	  $_SESSION['state'] = $state;

	  $authUrl = $client->createAuthUrl();
	}

$this->autoRender = false;
}