Update WebrtcFrameScheduler to avoid concurrent captures.

remoting/protocol/webrtc_frame_scheduler_simple.h

Index: remoting/protocol/webrtc_frame_scheduler_simple.h
diff --git a/remoting/protocol/webrtc_frame_scheduler_simple.h b/remoting/protocol/webrtc_frame_scheduler_simple.h
index 6a38b39dd1bab9f6f5778c5778ccd3b70b04d566..f5f3702df83799f825994ef240314acfe724b7db 100644
--- a/remoting/protocol/webrtc_frame_scheduler_simple.h
+++ b/remoting/protocol/webrtc_frame_scheduler_simple.h
@@ -16,9 +16,10 @@
 namespace remoting {
 namespace protocol {
 
-// WebrtcFrameSchedulerSimple is a simple implementation of
-// WebrtcFrameScheduler that always keeps only one frame in the pipeline.
-// It schedules each frame after the previous one is expected to finish sending.
+// WebrtcFrameSchedulerSimple is a simple implementation of WebrtcFrameScheduler
+// that always keeps only one frame in the pipeline. It schedules each frame
+// such that it is encoded and ready to be sent by the time previous one is
+// expected to finish sending.
 class WebrtcFrameSchedulerSimple : public VideoChannelStateObserver,
                                    public WebrtcFrameScheduler {
  public:
@@ -53,6 +54,9 @@ class WebrtcFrameSchedulerSimple : public VideoChannelStateObserver,
 
   LeakyBucket pacing_bucket_;
 
+  // Set to true when a frame is being captured or encoded.
+  bool frame_pending_ = false;
+
   // Set to true when encoding unchanged frames for top-off.
   bool top_off_is_active_ = false;