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This CL enables the following:

 - SupportedVideoDecoderConfigs are now returned from the GPU
   process as a map [video decoder implementation] => vector of
   SupportedVideoDecoderConfig for that implementation.
 - GpuVideoAcceleratorFactories::IsDecoderConfigSupported takes the
   implementation enum value as an additional argument, and checks
   only against that implementation's supported configs.
 - MojoVideoDecoder accepts an implementation enum during
   construction, which will be provided to the service during
   decoder initialization.
 - GpuMojoMediaClient uses the kAlternate implementation enum value
   to select D3D11VideoDecoder on Windows, if the experiment is
   currently enabled.  It no longer uses FallbackVideoDecoder.
 - MojoDecoderFactory adds a kAlternate decoder, in addition to the
   kDefault decoder, on Windows if the experiment is enabled.
 - RTCVideoDecoderAdapter always selects the default video decoder
   implementation.  This causes RTC on windows to use only DXVA,
   rather than D3D11VideoDecoder.  This will be addressed as follow-
   up work, though likely not immediately.

The main reason for this is that it lets the renderer's decoder
selection algorithm see two MojoVideoDecoder instances: one for DXVA
and one for D3D11VideoDecoder.  This lets decoder selection and
fallback happen normally for them, rather than having the fallback
happen in the GPU process via FallbackVideoDecoder.

Bug: 954422
Change-Id: If6f056723c11ff50327623ac8d554d9f5da9f499
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534984Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652479}