Validate references of frames inserted into FrameBuffer2.

BUG=chromium:730603

Review-Url: https://codereview.webrtc.org/2937243002
Cr-Commit-Position: refs/heads/master@{#18614}

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;