cast: support cursor rendering for tab capture

content/public/browser/render_widget_host_view_frame_subscriber.h

Index: content/public/browser/render_widget_host_view_frame_subscriber.h
diff --git a/content/public/browser/render_widget_host_view_frame_subscriber.h b/content/public/browser/render_widget_host_view_frame_subscriber.h
index 653316606d2ae429bd5c380c39904647b131de8d..b5448b449e79fda81804e517f343c227cfec729a 100644
--- a/content/public/browser/render_widget_host_view_frame_subscriber.h
+++ b/content/public/browser/render_widget_host_view_frame_subscriber.h
@@ -40,7 +40,11 @@ class RenderWidgetHostViewFrameSubscriber {
   // If |frame_captured| is false then the content in frame provided is
   // invalid. There was an error during the process of frame capture or the
   // platform layer is shutting down. |timestamp| is also invalid in this case.
+  //
+  // |region_in_frame| is the location within the Videoframe where the
+  // captured content resides, with the rest of the VideoFrame blacked out.
   typedef base::Callback<void(base::TimeTicks /* timestamp */,
+                              const gfx::Rect& /* region_in_frame */,
                               bool /* frame_captured */)> DeliverFrameCallback;
 
   // Called when a new frame is going to be presented at time