Clear the FrameBuffer in case of backward jumps in the picture id.

webrtc/modules/video_coding/frame_buffer2.h

Index: webrtc/modules/video_coding/frame_buffer2.h
diff --git a/webrtc/modules/video_coding/frame_buffer2.h b/webrtc/modules/video_coding/frame_buffer2.h
index b41ef2ff340e546517d48c2b12b65649561d9dc3..f667fd532caea94b9a5c2f3355f6036fa9dffb91 100644
--- a/webrtc/modules/video_coding/frame_buffer2.h
+++ b/webrtc/modules/video_coding/frame_buffer2.h
@@ -143,6 +143,8 @@ class FrameBuffer {
 
   void UpdateHistograms() const;
 
+  void ClearFramesAndHistory() EXCLUSIVE_LOCKS_REQUIRED(crit_);
+
   FrameMap frames_ GUARDED_BY(crit_);
 
   rtc::CriticalSection crit_;
@@ -151,6 +153,7 @@ class FrameBuffer {
   VCMJitterEstimator* const jitter_estimator_ GUARDED_BY(crit_);
   VCMTiming* const timing_ GUARDED_BY(crit_);
   VCMInterFrameDelay inter_frame_delay_ GUARDED_BY(crit_);
+  uint32_t last_decoded_frame_timestamp_ GUARDED_BY(crit_);
   FrameMap::iterator last_decoded_frame_it_ GUARDED_BY(crit_);
   FrameMap::iterator last_continuous_frame_it_ GUARDED_BY(crit_);
   int num_frames_history_ GUARDED_BY(crit_);