Add UMA histograms to MHTML save operations.

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 6756d31328f49b9eea67a885ade134a6280e9786..351f5302c2a0c45dd61d16b2a4a8f5e1bf63205f 100644
--- a/content/browser/frame_host/render_frame_host_impl.h
+++ b/content/browser/frame_host/render_frame_host_impl.h
@@ -726,7 +726,8 @@ class CONTENT_EXPORT RenderFrameHostImpl
   void OnSerializeAsMHTMLResponse(
       int job_id,
       bool success,
-      const std::set<std::string>& digests_of_uris_of_serialized_resources);
+      const std::set<std::string>& digests_of_uris_of_serialized_resources,
+      base::TimeDelta renderer_main_thread_time);
 
 #if defined(USE_EXTERNAL_POPUP_MENU)
   void OnShowPopup(const FrameHostMsg_ShowPopup_Params& params);