More gracefully handle timing errors, such as unexpected changes in the rtp timestamp.

BUG=webrtc:7682

Review-Url: https://codereview.webrtc.org/2898763005
Cr-Commit-Position: refs/heads/master@{#18245}

webrtc/modules/video_coding/frame_buffer2.h

@@ -150,6 +150,9 @@ class FrameBuffer {
 
   void ClearFramesAndHistory() EXCLUSIVE_LOCKS_REQUIRED(crit_);
 
+  bool HasBadRenderTiming(const FrameObject& frame, int64_t now_ms)
+      EXCLUSIVE_LOCKS_REQUIRED(crit_);
+
   FrameMap frames_ GUARDED_BY(crit_);
 
   rtc::CriticalSection crit_;