Attempt to fixed crash on stop mirroring by stopping render thread on a worker thread.

content/browser/renderer_host/media/video_capture_device_impl.h

Index: content/browser/renderer_host/media/video_capture_device_impl.h
diff --git a/content/browser/renderer_host/media/video_capture_device_impl.h b/content/browser/renderer_host/media/video_capture_device_impl.h
index d2c03f50a2cd1ad51f6b6f60836b3260093ace42..56bf1e0eac8dbd564f489de78df3146b33c01030 100644
--- a/content/browser/renderer_host/media/video_capture_device_impl.h
+++ b/content/browser/renderer_host/media/video_capture_device_impl.h
@@ -119,7 +119,8 @@ class VideoCaptureMachine {
       const scoped_refptr<ThreadSafeCaptureOracle>& oracle_proxy) = 0;
 
   // Stops capturing. Must be run on the UI BrowserThread.
-  virtual void Stop() = 0;
+  // |callback| is invoked after the capturing has stopped.
+  virtual void Stop(const base::Closure& callback) = 0;
 
  protected:
   bool started_;