Check for enable-accelerated-video-decode on Linux

Video decoding was being accelerated on Linux even though the newly
added "enable-accelerated-video-decode" flag was not specified. The
chrome://gpu page was misleadingly showing this feature as disabled:

  > Video Decode: Software only. Hardware acceleration disabled

This change adds a check for --enable-accelerated-video-decode when
considering if video decoding should be activated. (Only on Linux.)

Extra context: https://crbug.com/1097029#c18 (and also comment 20).

Bug: 1066176, 1097029
Change-Id: I534115f5f6ceed0ee3511fcf5c2d0f1dd04b9b7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431434Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811480}

content/renderer/render_thread_impl.cc

@@ -1121,7 +1121,11 @@ media::GpuVideoAcceleratorFactories* RenderThreadImpl::GetGpuFactories() {
           kGpuStreamPriorityMedia);
 
   const bool enable_video_accelerator =
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
+      cmd_line->HasSwitch(switches::kEnableAcceleratedVideoDecode) &&
+#else
       !cmd_line->HasSwitch(switches::kDisableAcceleratedVideoDecode) &&
+#endif  // defined(OS_LINUX) && !defined(OS_CHROMEOS)
       (gpu_channel_host->gpu_feature_info()
            .status_values[gpu::GPU_FEATURE_TYPE_ACCELERATED_VIDEO_DECODE] ==
        gpu::kGpuFeatureStatusEnabled);