getUserMeida: report device starting states

media/capture/video/mac/video_capture_device_mac.mm

Index: media/capture/video/mac/video_capture_device_mac.mm
diff --git a/media/capture/video/mac/video_capture_device_mac.mm b/media/capture/video/mac/video_capture_device_mac.mm
index d14f55102f8559442b74095a7db9fb25cd0af198..3477a5abd4139527f0eebe09a23de163469845e8 100644
--- a/media/capture/video/mac/video_capture_device_mac.mm
+++ b/media/capture/video/mac/video_capture_device_mac.mm
@@ -359,6 +359,7 @@ void VideoCaptureDeviceMac::AllocateAndStart(
     return;
   }
 
+  client_->OnStarted();
   state_ = kCapturing;
 }
 
@@ -406,6 +407,9 @@ void VideoCaptureDeviceMac::ReceiveFrame(const uint8_t* video_frame,
                                          int aspect_numerator,
                                          int aspect_denominator,
                                          base::TimeDelta timestamp) {
+  if (state_ != kCapturing)

[chfremer, 2017/02/17 23:51:15]

Unless this runs on the same thread as AllocateAndStart(), this check may happen
concurrently with AllocateAndStart() changing |state_| to kCapturing. I don't
know if this is a legal atomic operation or if it needs to be synchronized using
locks. Could you please check/confirm?

[braveyao, 2017/02/22 18:06:44]

Done.
Tried to apply std::atomic<> to all other places.

+    return;
+
   if (capture_format_.frame_size != frame_format.frame_size) {
     ReceiveError(FROM_HERE,
                  "Captured resolution " + frame_format.frame_size.ToString() +