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This is a WIP.  Basic playback works, though EOS is ignored.  There is
quite a bit of thread-hopping, since the accelerator / decoder is run
on the GPU main thread rather than the mojo thread.  This will be
eliminated in a later CL.

Resource cleanup is of the "and the process will exit anyway" variety.

It's based heavily on D3D11VideoDecodeAccelerator.  Re-uses the H264
Accelerated Video Decoder for D3D11.  The accelerator was written with
the VDA in mind, so the VideoDecoder jumps through some minor hoops
to use it also.  It is intended that this will replace the VDA as soon
as it reaches parity with it, and the accelerator can be modified for
easier use with the VideoDecoder.

Set "enable_d3d11_video_decoder=true" in gn config.  This will enable
the decoder, and:
 - enable MojoVideoDecoder
 - enable mojo CDM
 - set the media mojo host to be the GPU process.

Also incorporates fixes to the d3d11_h264_accelerator from:
https://chromium-review.googlesource.com/c/chromium/src/+/661237
by rkuroiwa@ .

Bug: 
Change-Id: Ie5f2b651895bc812e5e6bba7c5eb302f2702a1b6
Reviewed-on: https://chromium-review.googlesource.com/693284
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509615}