Enable mojo video decoders by default on Linux desktop if use_vaapi i…

…s 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}

media/media_options.gni

@@ -5,6 +5,7 @@
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
+import("//media/gpu/args.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 
 # Do not expand this list without double-checking with OWNERS, this is a list of
@@ -129,8 +130,9 @@ declare_args() {
   # |mojo_media_services|). When enabled, selected mojo paths will be enabled in
   # the media pipeline and corresponding services will hosted in the selected
   # remote process (e.g. "utility" process, see |mojo_media_host|).
-  enable_mojo_media = is_android || is_chromecast || is_chromeos || is_mac ||
-                      is_win || enable_library_cdms
+  enable_mojo_media =
+      is_android || is_chromecast || is_chromeos || is_mac || is_win ||
+      enable_library_cdms || (is_desktop_linux && use_vaapi)
 
   # Enable the TestMojoMediaClient to be used in mojo MediaService. This is for
   # testing only and will override the default platform MojoMediaClient, if any.
@@ -200,7 +202,8 @@ if (enable_mojo_media) {
       ]
       _default_mojo_media_host = "gpu"
     }
-  } else if (is_chromeos || is_mac || is_win) {
+  } else if (is_chromeos || is_mac || is_win ||
+             (is_desktop_linux && use_vaapi)) {
     _default_mojo_media_services = [ "video_decoder" ]
     _default_mojo_media_host = "gpu"
   }

media/mojo/services/gpu_mojo_media_client.cc

@@ -54,7 +54,7 @@ namespace media {
 namespace {
 
 #if defined(OS_ANDROID) || defined(OS_CHROMEOS) || defined(OS_MACOSX) || \
-    defined(OS_WIN)
+    defined(OS_WIN) || defined(OS_LINUX)
 gpu::CommandBufferStub* GetCommandBufferStub(
     base::WeakPtr<MediaGpuChannelManager> media_gpu_channel_manager,
     base::UnguessableToken channel_token,
@@ -148,7 +148,8 @@ std::unique_ptr<VideoDecoder> GpuMojoMediaClient::CreateVideoDecoder(
       android_overlay_factory_cb_, std::move(request_overlay_info_cb),
       std::make_unique<VideoFrameFactoryImpl>(gpu_task_runner_,
                                               std::move(get_stub_cb)));
-#elif defined(OS_CHROMEOS) || defined(OS_MACOSX) || defined(OS_WIN)
+#elif defined(OS_CHROMEOS) || defined(OS_MACOSX) || defined(OS_WIN) || \
+    defined(OS_LINUX)
   std::unique_ptr<VideoDecoder> vda_video_decoder = VdaVideoDecoder::Create(
       task_runner, gpu_task_runner_, media_log->Clone(), target_color_space,
       gpu_preferences_, gpu_workarounds_,