Fix off-by-one bugs in video_coding::PacketBuffer when the buffer is filled with a single frame.

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 fb2cd33f1830693a84643a6ba1cbd4d4537e6c3d..07510770417ef6f09a12cc0293bb32c186bab269 100644
--- a/webrtc/modules/video_coding/frame_object.h
+++ b/webrtc/modules/video_coding/frame_object.h
@@ -42,7 +42,7 @@ class FrameObject : public webrtc::VCMEncodedFrame {
   //                 been implemented.
   virtual bool delayed_by_retransmission() const { return 0; }
 
-  size_t size() { return _length; }
+  size_t size() const { return _length; }
 
   // The tuple (|picture_id|, |spatial_layer|) uniquely identifies a frame
   // object. For codec types that don't necessarily have picture ids they