Validate references of frames inserted into FrameBuffer2.

webrtc/modules/video_coding/frame_object.h

Index: webrtc/modules/video_coding/frame_object.h
diff --git a/webrtc/modules/video_coding/frame_object.h b/webrtc/modules/video_coding/frame_object.h
index 413cc3391036ac7a68e4c6d27466c1eac736cf6b..fa0a2c1d3d59859243e65144ae24437f5b86db6c 100644
--- a/webrtc/modules/video_coding/frame_object.h
+++ b/webrtc/modules/video_coding/frame_object.h
@@ -51,6 +51,8 @@ class FrameObject : public webrtc::VCMEncodedFrame {
   uint8_t spatial_layer;
   uint32_t timestamp;
 
+  // TODO(philipel): Add simple modify/access functions to prevent adding too
+  // many |references|.
   size_t num_references;
   uint16_t references[kMaxFrameReferences];
   bool inter_layer_predicted;