VP8Decoder: Drop references to reference frames earlier.

Drop references to reference frames before requesting new surfaces to
ensure we don't hold onto them while the Accelerator is changing resolution,
risking reusing a stale buffer.

BUG=chrome-os-partner:35539
TEST=Hangouts, vdatest

Review URL: https://codereview.chromium.org/880773009

Cr-Commit-Position: refs/heads/master@{#313679}

content/common/gpu/media/v4l2_slice_video_decode_accelerator.cc

@@ -1265,6 +1265,8 @@ void V4L2SliceVideoDecodeAccelerator::FinishSurfaceSetChangeIfNeeded() {
 
   DCHECK_EQ(state_, kIdle);
   DCHECK(decoder_display_queue_.empty());
+  // All output buffers should've been returned from decoder and device by now.
+  DCHECK_EQ(free_output_buffers_.size(), output_buffer_map_.size());
 
   // Keep input queue running while we switch outputs.
   if (!StopDevicePoll(true)) {

content/common/gpu/media/vp8_decoder.cc

@@ -75,6 +75,12 @@ VP8Decoder::DecodeResult VP8Decoder::Decode() {
     if (new_pic_size != pic_size_) {
       DVLOG(2) << "New resolution: " << new_pic_size.ToString();
       pic_size_ = new_pic_size;
+
+      DCHECK(!curr_pic_);
+      last_frame_ = nullptr;
+      golden_frame_ = nullptr;
+      alt_frame_ = nullptr;
+
       return kAllocateNewSurfaces;
     }