Support external buffer import in VDA interface and add a V4L2SVDA impl.

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 0109dc7dae39901d94d3fb7a493da6aa722941bd..6a84dede9800e8b69bff3787d3f70058908599e6 100644
--- a/media/filters/gpu_video_decoder.cc
+++ b/media/filters/gpu_video_decoder.cc
@@ -510,10 +510,8 @@ void GpuVideoDecoder::PictureReady(const media::Picture& picture) {
 
   DCHECK(decoder_texture_target_);
 
-  bool opaque = IsOpaque(config_.format());
-
   scoped_refptr<VideoFrame> frame(VideoFrame::WrapNativeTexture(
-      opaque ? PIXEL_FORMAT_XRGB : PIXEL_FORMAT_ARGB,
+      vda_->GetOutputFormat(),

[kcwu, 2016/03/22 07:47:02]

The original code will use XRGB for some cases. But new code seems no ways to
use XRGB.
Is this correct?

[Pawel Osciak, 2016/03/28 01:31:29]

If a VDA doesn't implement GetOutputFormat(), it will default to XRGB returned
by the implementation in the base class.

[Pawel Osciak, 2016/03/28 01:59:10]

Sorry, I meant ARGB. As for the choice between XRGB and ARGB, we used to have it
as ARGB only. First of all, the VDA has to report the format it produces, as it
is the only entity that knows about it. Further, I believe I discussed this
issue with original author of the change to allow selection, and I believe we
agreed this was preferred. I'll loop back the original author again for a
confirmation.

       gpu::MailboxHolder(pb.texture_mailbox(), gpu::SyncToken(),
                          decoder_texture_target_),
       BindToCurrentLoop(base::Bind(