Add UMA to measure potential impact of Fast Tab Close.

content/browser/web_contents/web_contents_impl.h

Index: content/browser/web_contents/web_contents_impl.h
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index 2fda9db2e0e02ae4a224db704aa8f206b00e0cf1..6f52a59d2ab05a0462a2ff9746067b87c04d96b6 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -426,7 +426,7 @@ class CONTENT_EXPORT WebContentsImpl
   virtual bool CreateRenderViewForRenderManager(
       content::RenderViewHost* render_view_host, int opener_route_id) OVERRIDE;
   virtual void BeforeUnloadFiredFromRenderManager(
-      bool proceed,
+      bool proceed, const base::TimeTicks& proceed_time,
       bool* proceed_to_fire_unload) OVERRIDE;
   virtual void RenderViewGoneFromRenderManager(
       content::RenderViewHost* render_view_host) OVERRIDE;
@@ -799,6 +799,9 @@ class CONTENT_EXPORT WebContentsImpl
   // The time that we started to close this WebContents.
   base::TimeTicks close_start_time_;
 
+  // The time when onbeforeunload ended.
+  base::TimeTicks before_unload_end_time_;
+
   // The time that this tab was last selected.
   base::TimeTicks last_selected_time_;