Fix: tabs slow to response to beforeunload are closed prematurely.

content/browser/frame_host/render_frame_host_impl.h

Index: content/browser/frame_host/render_frame_host_impl.h
diff --git a/content/browser/frame_host/render_frame_host_impl.h b/content/browser/frame_host/render_frame_host_impl.h
index 45d8ca0cffc75a6862df86dcbaef4b24a71fe007..bc020fb0e5133d0fd0d65d894f810a2aa2e679f5 100644
--- a/content/browser/frame_host/render_frame_host_impl.h
+++ b/content/browser/frame_host/render_frame_host_impl.h
@@ -401,6 +401,9 @@ class CONTENT_EXPORT RenderFrameHostImpl : public RenderFrameHost,
   // PlzNavigate: this call happens on all browser-initiated navigations.
   void DispatchBeforeUnload(bool for_navigation);
 
+  // Simulate beforeunload ack on behalf of renderer if it's unrenresponsive.
+  void SimulateBeforeUnloadAck();
+
   // Returns true if a call to DispatchBeforeUnload will actually send the
   // BeforeUnload IPC. This is the case if the current renderer is live and this
   // frame is the main frame.