Move beforeunload hang timer duties to its own timer.

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 c59708b16aa2b515e6812f3d192bf3548e2e187f..b58f1ac7591bc01df6c829e9609bb24a72d024d1 100644
--- a/content/browser/frame_host/render_frame_host_impl.h
+++ b/content/browser/frame_host/render_frame_host_impl.h
@@ -458,7 +458,6 @@ class CONTENT_EXPORT RenderFrameHostImpl
   void JavaScriptDialogClosed(IPC::Message* reply_msg,
                               bool success,
                               const base::string16& user_input,
-                              bool is_before_unload_dialog,
                               bool dialog_was_suppressed);
 
   // Get the accessibility mode from the delegate and Send a message to the
@@ -888,6 +887,9 @@ class CONTENT_EXPORT RenderFrameHostImpl
   std::unique_ptr<NavigationHandleImpl> TakeNavigationHandleForCommit(
       const FrameHostMsg_DidCommitProvisionalLoad_Params& params);
 
+  // Called by |beforeunload_timeout_| when the beforeunload timeout fires.
+  void BeforeUnloadTimeout();
+
   // For now, RenderFrameHosts indirectly keep RenderViewHosts alive via a
   // refcount that calls Shutdown when it reaches zero.  This allows each
   // RenderFrameHostManager to just care about RenderFrameHosts, while ensuring
@@ -992,6 +994,11 @@ class CONTENT_EXPORT RenderFrameHostImpl
   // PlzNavigate: all navigations require a beforeUnload ACK.
   bool unload_ack_is_for_navigation_;
 
+  // The timeout monitor that runs from when the beforeunload is started in
+  // DispatchBeforeUnload() until either the render process ACKs it with an IPC
+  // to OnBeforeUnloadACK(), or until the timeout triggers.
+  std::unique_ptr<TimeoutMonitor> beforeunload_timeout_;
+
   // Indicates whether this RenderFrameHost is in the process of loading a
   // document or not.
   bool is_loading_;