Revert of [chromecast] Pass media task runner to MediaPipelineBackendManager.

chromecast/browser/cast_content_browser_client.h

Index: chromecast/browser/cast_content_browser_client.h
diff --git a/chromecast/browser/cast_content_browser_client.h b/chromecast/browser/cast_content_browser_client.h
index 5f0d77eab96d9a74092d0f2c82070b59b81961fe..19ccffacc522ab4e15ac669d140e9b70826791ca 100644
--- a/chromecast/browser/cast_content_browser_client.h
+++ b/chromecast/browser/cast_content_browser_client.h
@@ -33,7 +33,6 @@
 
 namespace media {
 class MediaPipelineBackend;
-class MediaPipelineBackendManager;
 struct MediaPipelineDeviceParams;
 class MediaResourceTracker;
 class VideoPlaneController;
@@ -74,8 +73,6 @@
       const media::MediaPipelineDeviceParams& params);
 
   media::MediaResourceTracker* media_resource_tracker();
-
-  media::MediaPipelineBackendManager* media_pipeline_backend_manager();
 #endif
 
   // Invoked when the metrics client ID changes.