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

content/browser/frame_host/render_frame_host_impl.cc

Index: content/browser/frame_host/render_frame_host_impl.cc
diff --git a/content/browser/frame_host/render_frame_host_impl.cc b/content/browser/frame_host/render_frame_host_impl.cc
index de78df1b008a679665336147ce250c5ad9bf3a98..a9ed5518369cdb73b6c48b144a73e96e4f59260b 100644
--- a/content/browser/frame_host/render_frame_host_impl.cc
+++ b/content/browser/frame_host/render_frame_host_impl.cc
@@ -2124,6 +2124,12 @@ void RenderFrameHostImpl::DispatchBeforeUnload(bool for_navigation) {
   }
 }
 
+void RenderFrameHostImpl::SimulateBeforeUnloadAck() {
+  DCHECK(is_waiting_for_beforeunload_ack_);
+  base::TimeTicks approx_renderer_start_time = send_before_unload_start_time_;
+  OnBeforeUnloadACK(true, approx_renderer_start_time, base::TimeTicks::Now());
+}
+
 bool RenderFrameHostImpl::ShouldDispatchBeforeUnload() {
   // TODO(creis): Support beforeunload on subframes.
   return !GetParent() && IsRenderFrameLive();