use lock to make sure device will not deliver any frame after it's stopped.

media/video/capture/mac/video_capture_device_mac.mm

Index: media/video/capture/mac/video_capture_device_mac.mm
===================================================================
--- media/video/capture/mac/video_capture_device_mac.mm	(revision 155213)
+++ media/video/capture/mac/video_capture_device_mac.mm	(working copy)
@@ -49,6 +49,7 @@
 
 void VideoCaptureDeviceMac::Allocate(int width, int height, int frame_rate,
                                      EventHandler* observer) {
+  base::AutoLock auto_lock(lock_);
   if (state_ != kIdle) {
     return;
   }
@@ -85,16 +86,19 @@
     SetErrorState("Could not start capture device.");
     return;
   }
+  base::AutoLock auto_lock(lock_);
   state_ = kCapturing;
 }
 
 void VideoCaptureDeviceMac::Stop() {
   DCHECK_EQ(state_, kCapturing);
   [capture_device_ stopCapture];
+  base::AutoLock auto_lock(lock_);
   state_ = kAllocated;
 }
 
 void VideoCaptureDeviceMac::DeAllocate() {
+  base::AutoLock auto_lock(lock_);
   if (state_ != kAllocated && state_ != kCapturing) {
     return;
   }
@@ -134,12 +138,16 @@
     const uint8* video_frame,
     int video_frame_length,
     const VideoCaptureCapability& frame_info) {
-  observer_->OnIncomingCapturedFrame(video_frame, video_frame_length,
-                                     base::Time::Now());
+  base::AutoLock auto_lock(lock_);
+  if (state_ == kCapturing) {
+    observer_->OnIncomingCapturedFrame(video_frame, video_frame_length,
+                                       base::Time::Now());
+  }
 }
 
 void VideoCaptureDeviceMac::SetErrorState(const std::string& reason) {
   DLOG(ERROR) << reason;
+  base::AutoLock auto_lock(lock_);
   state_ = kError;
   observer_->OnError();
 }