Add UMA counter for hung renderers on cross-site navigation.

content/browser/renderer_host/render_view_host_impl.h

Index: content/browser/renderer_host/render_view_host_impl.h
diff --git a/content/browser/renderer_host/render_view_host_impl.h b/content/browser/renderer_host/render_view_host_impl.h
index ee52dde970fff02c7a41fe955a84cdb3e1c4a638..46ba91072456f429cb61d9b36124adf90341c64a 100644
--- a/content/browser/renderer_host/render_view_host_impl.h
+++ b/content/browser/renderer_host/render_view_host_impl.h
@@ -649,6 +649,9 @@ class CONTENT_EXPORT RenderViewHostImpl
   // is_waiting_for_beforeunload_ack_, unload_ack_is_for_cross_site_transition_.
   bool is_waiting_for_unload_ack_;
 
+  // Set to true when waiting for ViewHostMsg_SwapOut_ACK has timed out.
+  bool has_timed_out_on_unload_;
+
   // Valid only when is_waiting_for_beforeunload_ack_ or
   // is_waiting_for_unload_ack_ is true.  This tells us if the unload request
   // is for closing the entire tab ( = false), or only this RenderViewHost in