Add UMA histograms to MHTML save operations.

content/common/frame_messages.h

Index: content/common/frame_messages.h
diff --git a/content/common/frame_messages.h b/content/common/frame_messages.h
index b9520d9ae87df077d5cedaa5df0eb4ec34f66509..9e609ede9244c3cf587cb14e6352995339ee525a 100644
--- a/content/common/frame_messages.h
+++ b/content/common/frame_messages.h
@@ -1497,11 +1497,12 @@ IPC_MESSAGE_ROUTED2(FrameHostMsg_SerializedHtmlWithLocalLinksResponse,
                     bool /* end of data? */)
 
 // Response to FrameMsg_SerializeAsMHTML.
-IPC_MESSAGE_ROUTED3(
+IPC_MESSAGE_ROUTED4(
     FrameHostMsg_SerializeAsMHTMLResponse,
     int /* job_id (used to match responses to requests) */,
     bool /* true if success, false if error */,
-    std::set<std::string> /* digests of uris of serialized resources */)
+    std::set<std::string> /* digests of uris of serialized resources */,
+    base::TimeDelta /* how much time of the main render thread was used */)
 
 // Sent when the renderer updates hint for importance of a tab.
 IPC_MESSAGE_ROUTED1(FrameHostMsg_UpdatePageImportanceSignals,