ImageCapture: avoid several grabFrame()s in flight, beef up content_browsertests

content/renderer/media/image_capture_frame_grabber.h

Index: content/renderer/media/image_capture_frame_grabber.h
diff --git a/content/renderer/media/image_capture_frame_grabber.h b/content/renderer/media/image_capture_frame_grabber.h
index df336267cf0f7821183cfa21a6aa42adad660c9f..5b0fcbf6f861a9fc003e2dde6963c70393ca8eb3 100644
--- a/content/renderer/media/image_capture_frame_grabber.h
+++ b/content/renderer/media/image_capture_frame_grabber.h
@@ -46,6 +46,9 @@ class CONTENT_EXPORT ImageCaptureFrameGrabber final
       ScopedWebCallbacks<blink::WebImageCaptureGrabFrameCallbacks> callbacks,
       sk_sp<SkImage> image);
 
+  // Flag to indicate that there is a frame grabbing in progress.
+  bool frame_grab_in_progress_;
+
   base::ThreadChecker thread_checker_;
   base::WeakPtrFactory<ImageCaptureFrameGrabber> weak_factory_;