getUserMeida: report device starting states

media/capture/video/video_capture_device.h

Index: media/capture/video/video_capture_device.h
diff --git a/media/capture/video/video_capture_device.h b/media/capture/video/video_capture_device.h
index a716db8665967b41a6af20cf42600324457ff978..5c706e0960160510c3168b903447b3e88fae1f06 100644
--- a/media/capture/video/video_capture_device.h
+++ b/media/capture/video/video_capture_device.h
@@ -84,6 +84,8 @@ class CAPTURE_EXPORT VideoCaptureDevice
   // is actually two-in-one: clients may implement OnIncomingCapturedData() or
   // ReserveOutputBuffer() + OnIncomingCapturedVideoFrame(), or all of them.
   // All clients must implement OnError().
+  // All clients must implement OnStarted(). Captured frames can be delivered
+  // as soon as AllocateAndStart() is called, despite of OnStarted processing.

[chfremer, 2017/02/22 19:13:58]

As discussed offline, the "All clients must implement ..." is not meaningful,
since the language already enforces that purely virtual methods must be
implemented. Please remove that part.

The "despite of OnStarted processing" part is hard to understand. I would write
something like this: 
All methods may be called as soon as AllocateAndStart() of the corresponding
VideoCaptureDevice is invoked. The methods for buffer reservation and frame
delivery may be called from arbitrary threads but are guaranteed to be called
non-concurrently. The status reporting methods (OnStarted, OnLog, OnError) may
be called concurrently.

[braveyao, 2017/02/23 00:06:26]

Done.

   class CAPTURE_EXPORT Client {
    public:
     // Move-only type representing access to a buffer handle as well as
@@ -214,6 +216,12 @@ class CAPTURE_EXPORT VideoCaptureDevice
     // Returns the current buffer pool utilization, in the range 0.0 (no buffers
     // are in use by producers or consumers) to 1.0 (all buffers are in use).
     virtual double GetBufferPoolUtilization() const = 0;
+
+    // VideoCaptureDevice reports it's successfully started.
+    // This method may be called either synchronously from an invocation of
+    // VideoCaptureDevice::AllocateAndStart() or asynchronously from any thread
+    // after AllocateAndStart() has returned.

[chfremer, 2017/02/22 19:13:58]

According to the latest design, it may actually be called even _before_
AllocateAndStart() has returned. It may be called as soon as AllocateAndStart()
is invoked.

And since we are now explaining the contract of when this may be called in the
class description (at the top), we no longer need to specify it here. Please
remove.

[braveyao, 2017/02/23 00:06:26]

Done.

+    virtual void OnStarted() = 0;
   };
 
   ~VideoCaptureDevice() override;