Eliminate video capture thread in renderer

Eliminate video capture thread in renderer

The main motivation of this change is to remove the video capture thread
in the renderer. All users of a video capture device already handles the
video frame on their thread. There is no need to call the clients with
an additional thread.

Summary of this change:
* Video capture thread eliminated
  VideoCaptureImpl now runs on the IO thread. Clients are called on the
  IO thread.
* Simplified VideoCaptureImplManager
  We still need to keep this object for the purpose of sharing a
  VideoCaptureImpl object with multiple clients. It should own these
  objects and maintain the usage count. A couple clean up items are done
  on this class:
  * It doesn't own the video capture thread now.
  * It is now a render thread only object.
  * It maintains refcount of a VideoCaptureImpl explicitly.
  * It is no longer refcounted.
  * Clients access it through RenderThreadImpl. Which ensures usage is
    on the render thread.
* New VideoCaptureHandle class
  Object of this class is returned by VideoCaptureImplManager to give
  access to a media::VideoCapture object. It is purely a wrapper and
  helps to do refcounting on the render thread.

Testing:
Added unit tests for VideoCaptureImplManager to test refcounting.
Also updated unit test for VideoCaptureImpl due to the threading
changes.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=244074

Review URL: https://codereview.chromium.org/120893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244687 0039d316-1c4b-4281-b951-d872f2087c98