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..a3c0166372582f13e3e8665c74e43ebcee8739e3 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();

[chfremer, 2017/02/16 01:00:50]

Unless ReceiveFrame() is invoked on the same thread as this one, this looks like
a race condition, i.e. it could happen that ReceiveFrame() is called before
OnStarted().

Note, that ReceiveFrame() does not check |state_| in order to discard frames
until |state_ == kCapturing|.

Any reason to not call client_->OnStarted() before [capture_device_
startCapture]?

[braveyao, 2017/02/17 20:37:21]

OnStarted must be called at the last of AllocatAndStart to ensure the device is
successfully started.

   state_ = kCapturing;
 }