[Sync] Add tab sync latency UMA

chrome/browser/ui/webui/foreign_session_handler.h

Index: chrome/browser/ui/webui/foreign_session_handler.h
diff --git a/chrome/browser/ui/webui/foreign_session_handler.h b/chrome/browser/ui/webui/foreign_session_handler.h
index 81e96ab9ed415f823fca28a991a0e2fbf17a506e..c0d019565c89fa507df94395de5194dd7700bac1 100644
--- a/chrome/browser/ui/webui/foreign_session_handler.h
+++ b/chrome/browser/ui/webui/foreign_session_handler.h
@@ -94,6 +94,10 @@ class ForeignSessionHandler : public content::WebUIMessageHandler,
   // The Registrar used to register ForeignSessionHandler for notifications.
   content::NotificationRegistrar registrar_;
 
+  // The time at which this WebUI was created. Used to calculate how long
+  // the WebUI was present before the sessions data was visible.
+  base::TimeTicks load_attempt_time_;
+
   DISALLOW_COPY_AND_ASSIGN(ForeignSessionHandler);
 };