VideoStreamDecoderImpl implementation, part 1.

In this CL the OnFrame function is implemented.

Bug: webrtc:8909
Change-Id: I68488a033e86eadd0b16d091faad14e9cda7cc36
Reviewed-on: https://webrtc-review.googlesource.com/64121
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22583}

modules/video_coding/frame_buffer2.h

@@ -47,6 +47,7 @@ class FrameBuffer {
 
   // Insert a frame into the frame buffer. Returns the picture id
   // of the last continuous frame or -1 if there is no continuous frame.
+  // TODO(philipel): Return a VideoLayerFrameId and not only the picture id.
   int64_t InsertFrame(std::unique_ptr<EncodedFrame> frame);
 
   // Get the next frame for decoding. Will return at latest after