Add OnRemoved() in VideoCapture::EventHandler API

media/video/capture/video_capture.h

Index: media/video/capture/video_capture.h
===================================================================
--- media/video/capture/video_capture.h	(revision 102835)
+++ media/video/capture/video_capture.h	(working copy)
@@ -66,6 +66,10 @@
     // Notify client that video capture has hit some error |error_code|.
     virtual void OnError(VideoCapture* capture, int error_code) = 0;
 
+    // Notify client that the client has been removed and no more calls will be
+    // received.
+    virtual void OnRemoved(VideoCapture* capture) = 0;
+
     // Notify client that a buffer is available.
     virtual void OnBufferReady(VideoCapture* capture,
                                scoped_refptr<VideoFrameBuffer> buffer) = 0;
@@ -93,10 +97,12 @@
 
   // Request video capture to start capturing with |capability|.
   // Also register |handler| with video capture for event handling.
+  // |handler| must remain valid until it has received |OnRemoved()|.
   virtual void StartCapture(EventHandler* handler,
                             const VideoCaptureCapability& capability) = 0;
 
   // Request video capture to stop capturing for client |handler|.
+  // |handler| must remain valid until it has received |OnRemoved()|.
   virtual void StopCapture(EventHandler* handler) = 0;
 
   // Feed buffer to video capture when done with it.