Revert of Remove threading from RendererGpuVideoAcceleratorFactories (https://codereview.chromium.org/27420004/)

Reason for revert:
Sorry, it looks like you broke the Win 7 Tests (dbg)(2) bot. http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%282%29/builds/19785

Original issue's description:
> This change removes all the threading considerations from
> GpuVideoAcceleratorFactories (and its implementation,
> RendererGpuVideoAcceleratorFactories).  Most notably, it removes Abort() and
> associated functions and state.  And with the removal of Abort() and friends,
> we can also remove its Clone() interface.
> 
> All of the previously abortable operations on the RGVAF (with the exception of
> ReadPixels()) can be made non-abortable, with no functional difference, due to
> the way the users of RGVAF function.  These three users are
> WebMediaPlayerImpl/GpuVideoDecoder, RTCVideoDecoder, and RTCVideoEncoder, and
> they can be made non-abortable because:
> 
> WebMediaPlayerImpl/GpuVideoDecoder:
> * Abort() is called from WebMediaPlayerImpl::Destroy().  It has no effect, as:
>   * All the RGVAF entry points are called from the the RGVAF message loop
>     from GpuVideoDecoder (except for ReadPixels()), so the Abort() has no
>     effect on them.
> 
> RTCVideoDecoder:
> * Abort() is called from RTCVideoDecoder::WillDestroyCurrentMessageLoop() for
>   the RGVAF message loop.  It has no effect, as:
>   * Amost all the RGVAF entry points are called from the RGVAF message loop
>     (except for ReadPixels()), so Abort() has no effect on them.
>   * The other exception is CreateVideoDecodeAccelerator(), which is called from
>     RTC's main thread.  But as the Abort() is called from
>     WillDestroyCurrentMessageLoop() for the RGVAF message loop itself, it is
>     guaranteed to occur after any tasks posted to the RGVAF message loop by
>     CreateVideoDecodeAccelerator() has completed, and so the Abort() has no
>     effect.
> 
> RTCVideoEncoder:
> * Abort() is called from RTCVideoDecoder::Release().  It has no effect, as:
>   * All the RGVAF entry points are called from the RGVAF message loop.
> 
> The only functional difference remaining is that making ReadPixels()
> non-abortable.  This is preferable, as as long as a completed video accelerator
> texture is available, it should be readable.  We also specify that all calls to
> ReadPixels must be made on the RGVAF message loop, like all other entry points,
> and leave it up to the users of ReadPixels() to handle thread trampolining if
> necessary.
> 
> BUG=306333
> TEST=local build, run on CrOS snow; build, run unittests on desktop Linux
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=247480

TBR=fischman@chromium.org,wuchengli@chromium.org,jamesr@chromium.org,jam@chromium.org,hshi@chromium.org,sheu@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=306333

Review URL: https://codereview.chromium.org/145103004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247655 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247655