Revert of Cast: Fix threading issues in VideoEncoderImpl (https://codereview.chromium.org/138953009/)

Reason for revert:
build break.

Original issue's description:
> Cast: Fix threading issues in VideoEncoderImpl
> 
> VideoEncoderImpl can be used after delete. We assumed that main thread
> out-live video encoder thread. This is not true for some use cases. For
> example in the renderer when video encoder thread is shared by multiple
> instances of CastSEnder. To fix this we need to separate Vp8Encoder and
> VideoEncoderImpl.
> 
> VideoEncoderImpl will live on the main thread only. And Vp8Encoder will
> live on the video encoder thread only.
> 
> Tested this change with vagrind and cast_unittests reports no error.
> 
> TBR=mikhal@chromium.org
> BUG=336887
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=246974

TBR=mikhal@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=336887

Review URL: https://codereview.chromium.org/132283006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246986 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246986