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

components/app_modal/javascript_app_modal_dialog.h

Index: components/app_modal/javascript_app_modal_dialog.h
diff --git a/components/app_modal/javascript_app_modal_dialog.h b/components/app_modal/javascript_app_modal_dialog.h
index 5c81ef7838ee839ba1d5ee67be98bb7c485e10c1..5383340e3950c25976b1953005b4f606d8913bc9 100644
--- a/components/app_modal/javascript_app_modal_dialog.h
+++ b/components/app_modal/javascript_app_modal_dialog.h
@@ -10,6 +10,7 @@
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/time/time.h"
 #include "components/app_modal/app_modal_dialog.h"
 #include "content/public/browser/javascript_dialog_manager.h"
 
@@ -102,6 +103,8 @@ class JavaScriptAppModalDialog : public AppModalDialog {
   base::string16 override_prompt_text_;
   bool use_override_prompt_text_;
 
+  base::TimeTicks creation_time_;
+
   DISALLOW_COPY_AND_ASSIGN(JavaScriptAppModalDialog);
 };