media/mojo: Remove fallback of ChromeOS VideoDecoder.

We plan to enable new VideoDecoder implementation per board, instead
of enable all boards at the same time. We won't enable boards that are
not supported yet.
This CL remove the fallback video decoder. Then we could catch any
initialize or runtime failure of new VideoDecoder implementation.

BUG=chromium:984482
TEST=open crosvideo.appspot.com on Kevin and Eve

Change-Id: I03026057b317ffde5436d8eb53c579ed18210bd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1734833Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684211}

media/mojo/services/gpu_mojo_media_client.cc

@@ -221,7 +221,6 @@ std::unique_ptr<VideoDecoder> GpuMojoMediaClient::CreateVideoDecoder(
               MaybeRenderEarlyManager::Create(gpu_task_runner_)));
 
 #elif defined(OS_CHROMEOS)
-      std::unique_ptr<VideoDecoder> cros_video_decoder;
       if (base::FeatureList::IsEnabled(kChromeosVideoDecoder)) {
 #if BUILDFLAG(USE_V4L2_CODEC) || BUILDFLAG(USE_VAAPI)
         auto frame_pool = std::make_unique<PlatformVideoFramePool>();
@@ -233,23 +232,11 @@ std::unique_ptr<VideoDecoder> GpuMojoMediaClient::CreateVideoDecoder(
                            media_gpu_channel_manager_,
                            command_buffer_id->channel_token,
                            command_buffer_id->route_id));
-        cros_video_decoder = ChromeosVideoDecoderFactory::Create(
+        video_decoder = ChromeosVideoDecoderFactory::Create(
             task_runner, std::move(frame_pool), std::move(frame_converter));
 #endif  // BUILDFLAG(USE_V4L2_CODEC) || BUILDFLAG(USE_VAAPI)
       }
 
-      if (cros_video_decoder) {
-        video_decoder = std::move(cros_video_decoder);
-      } else {
-        video_decoder = VdaVideoDecoder::Create(
-            task_runner, gpu_task_runner_, media_log->Clone(),
-            target_color_space, gpu_preferences_, gpu_workarounds_,
-            base::BindRepeating(&GetCommandBufferStub, gpu_task_runner_,
-                                media_gpu_channel_manager_,
-                                command_buffer_id->channel_token,
-                                command_buffer_id->route_id));
-      }
-
 #elif defined(OS_MACOSX) || defined(OS_WIN) || defined(OS_LINUX)
       video_decoder = VdaVideoDecoder::Create(
           task_runner, gpu_task_runner_, media_log->Clone(), target_color_space,