Fix race condition in FrameBuffer2

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 f667fd532caea94b9a5c2f3355f6036fa9dffb91..529428d8e7ab180bfd61a0e89d3f2ced3d2cde27 100644
--- a/webrtc/modules/video_coding/frame_buffer2.h
+++ b/webrtc/modules/video_coding/frame_buffer2.h
@@ -156,6 +156,7 @@ class FrameBuffer {
   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_);
+  FrameMap::iterator next_frame_it_ GUARDED_BY(crit_);
   int num_frames_history_ GUARDED_BY(crit_);
   int num_frames_buffered_ GUARDED_BY(crit_);
   bool stopped_ GUARDED_BY(crit_);