Enable mojo video decoders by default on Linux desktop if use_vaapi is true

Already the case for ChromeOS, Mac and Win. And run the service
in the GPU process too. Except that here the gn arg use_vaapi
has to be true as well.

Note that this CL does not change the following:
  - the gn arg 'use_vaapi' is still false by default on Linux,
    see media/gpu/args.gni
  - 'accelerated_video_decode' is still black listed on Linux,
    see entry 48 in gpu/config/software_rendering_list.json
  - it is still not possible to enable hw video decode from
    about:flags, see chrome/browser/about_flags.cc

Also note that with this CL the ffmpeg and libvpx video decoders
are still selected thanks to media::DecoderSelector::SelectDecoder
in case vaapi fails to initialize.

Also see https://chromium-review.googlesource.com/c/chromium/src/+/1225275/
which was very similar but for ChromeOS.

Tested on Linux desktop with gn args:
  - use_vaapi = true (default is false)
  ./out/release/chrome --ignore-gpu-blacklist --use-gl=desktop url_to_vp9_video
  ./out/release/chrome --ignore-gpu-blacklist --use-gl=egl url_to_vp9_video
   -> MojoVideoDecoder was in use and VaapiVideoDecodeAccelerator runing in the
      GPU process, through MojoVideoDecoderService

Bug: 522298
Change-Id: Ia19f9f3edc0af488a477a16001b7de4f4818b3b2
Reviewed-on: https://chromium-review.googlesource.com/c/1370717
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Julien Isorce <julien.isorce@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616901}
2 files changed