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..e1855edf97262b677513a9ff074f0e9fd76e424c 100644
--- a/media/capture/video/video_capture_device.h
+++ b/media/capture/video/video_capture_device.h
@@ -214,6 +214,9 @@ 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.

[miu, 2017/02/09 22:21:49]

There are two things you'll want to consider here:

1. Any thread can call OnStarted() (at least, FWICT in this patch). Consider
whether this is acceptable, or that OnStarted() must be called on the same
thread that called AllocateAndStart().

2. OnStarted() may be called synchronously (i.e., within a call to
AllocateAndStart()), or asynchronously. Are your client impl's prepared for
that?

Regardless of whether you decide to change (or not change) anything here, please
document these behaviors in the method comments here.

[chfremer, 2017/02/09 23:34:52]

Regardless of which exact thread it comes from, we should guarantee two things:
1.) OnStarted() is not called concurrently with any of the other methods
(actually, none of the VideoCaptureDevice::Client methods should be called
concurrently)
2.) OnStarted() is guaranteed to get invoked before the first frame is
delivered.

[braveyao, 2017/02/14 01:05:49]

All the events will be posted to IO thread. See
https://cs.chromium.org/chromium/src/content/browser/renderer_host/media/vide....
Please let me know if there is any more concern.

[braveyao, 2017/02/14 01:05:49]

All the methods will be posted to IO thread
(https://cs.chromium.org/chromium/src/content/browser/renderer_host/media/vide...).
So there won't be concurrent calls.
The state machine will guarantee that the first frame won't be  processed until
OnStarted is received. (see
https://cs.chromium.org/chromium/src/content/renderer/media/video_capture_imp...)

Please let me know if you have more concerns.

[chfremer, 2017/02/14 19:21:22]

Please see my follow-up comment in Patch Set 2.

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