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

BUG=webrtc:8028

Review-Url: https://codereview.webrtc.org/2993513002
Cr-Commit-Position: refs/heads/master@{#19209}

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