GpuVideoDecoder should reject playback of videos with alpha channel.

media/filters/gpu_video_decoder.cc

Index: media/filters/gpu_video_decoder.cc
diff --git a/media/filters/gpu_video_decoder.cc b/media/filters/gpu_video_decoder.cc
index 9627a4b156523c5ab3eaa72a881cf893d566d097..7a60aca09963a4bc6696c9d9dc38f1b812564a94 100644
--- a/media/filters/gpu_video_decoder.cc
+++ b/media/filters/gpu_video_decoder.cc
@@ -184,6 +184,12 @@ void GpuVideoDecoder::Initialize(const VideoDecoderConfig& config,
     return;
   }
 
+  if (config.format() == PIXEL_FORMAT_YV12A) {

[sandersd (OOO until July 31), 2016/06/17 18:09:19]

Please add a TODO to move this to capabilities, since it is conceivable that
some platforms could support such things, or to expose it in VDA::Config so that
each VDA can decide.

Not also that VDA::Config recently got |supported_output_formats|, but its use
is the inverse of this.

[DaleCurtis, 2016/06/17 18:19:56]

Done.

+    DVLOG(1) << "Alpha transparency formats are not supported.";
+    bound_init_cb.Run(false);
+    return;
+  }
+
   VideoDecodeAccelerator::Capabilities capabilities =
       factories_->GetVideoDecodeAcceleratorCapabilities();
   if (!IsProfileSupported(capabilities, config.profile(), config.coded_size(),