Add UMA histograms to track very brief or frequent tabs and JS dialogs.

components/app_modal/javascript_dialog_manager.h

Index: components/app_modal/javascript_dialog_manager.h
diff --git a/components/app_modal/javascript_dialog_manager.h b/components/app_modal/javascript_dialog_manager.h
index b204aaf8f210c25109a16309c01c24234c456fc3..9d70bd329d701bd5eb0a410bb359108b43b8f0ec 100644
--- a/components/app_modal/javascript_dialog_manager.h
+++ b/components/app_modal/javascript_dialog_manager.h
@@ -8,6 +8,7 @@
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/singleton.h"
+#include "base/time/time.h"
 #include "components/app_modal/javascript_app_modal_dialog.h"
 #include "content/public/browser/javascript_dialog_manager.h"
 
@@ -80,6 +81,11 @@ class JavaScriptDialogManager : public content::JavaScriptDialogManager {
   scoped_ptr<JavaScriptNativeDialogFactory> native_dialog_factory_;
   scoped_ptr<JavaScriptDialogExtensionsClient> extensions_client_;
 
+  // Record a single create and close timestamp to track the time between
+  // dialogs. (Since Javascript dialogs are modal, this is even accurate!)
+  base::TimeTicks last_close_time_;
+  base::TimeTicks last_creation_time_;
+
   DISALLOW_COPY_AND_ASSIGN(JavaScriptDialogManager);
 };