FrameBuffer2 now has Start/Stop methods.

The Stop method is used to signal any thread that is waiting in the
NextFrame function and will cause it to return immediately.

BUG=webrtc:5514
R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/2105323002 .

Cr-Commit-Position: refs/heads/master@{#13349}
diff --git a/webrtc/modules/video_coding/frame_buffer2.h b/webrtc/modules/video_coding/frame_buffer2.h
index 10cae42..3201f62 100644
--- a/webrtc/modules/video_coding/frame_buffer2.h
+++ b/webrtc/modules/video_coding/frame_buffer2.h
@@ -46,6 +46,14 @@
   // unique ptr if there is no available frame for decoding.
   std::unique_ptr<FrameObject> NextFrame(int64_t max_wait_time_ms);
 
+  // Start the frame buffer, has no effect if the frame buffer is started.
+  // The frame buffer is started upon construction.
+  void Start();
+
+  // Stop the frame buffer, causing any sleeping thread in NextFrame to
+  // return immediately.
+  void Stop();
+
  private:
   // FrameKey is a pair of (picture id, spatial layer).
   using FrameKey = std::pair<uint16_t, uint8_t>;
@@ -73,6 +81,7 @@
   VCMJitterEstimator* const jitter_estimator_;
   const VCMTiming* const timing_;
   int newest_picture_id_ GUARDED_BY(crit_);
+  bool stopped_ GUARDED_BY(crit_);
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(FrameBuffer);
 };