Add UMA metrics to measure the dismissal cause of dialogs.

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h

Index: chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h
diff --git a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h
index 839f0116fcb05be10563f340a7c125e50b76c3c5..f1ab74ba40859e5ba85c36710d4d3e45623810c5 100644
--- a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h
+++ b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h
@@ -66,6 +66,9 @@ class JavaScriptDialogTabHelper
 
  private:
   friend class content::WebContentsUserData<JavaScriptDialogTabHelper>;
+  enum class DismissalCause;
+
+  void LogDialogDismissalCause(DismissalCause cause);
 
   // Wrapper around a DialogClosedCallback so that we can intercept it before
   // passing it onto the original callback.
@@ -75,13 +78,18 @@ class JavaScriptDialogTabHelper
 
   void CloseDialog(bool suppress_callback,
                    bool success,
-                   const base::string16& user_input);
+                   const base::string16& user_input,
+                   DismissalCause cause);
 
   void ClearDialogInfo();
 
   // The dialog being displayed on the observed WebContents.
   base::WeakPtr<JavaScriptDialog> dialog_;
 
+  // The type of dialog being displayed. Only valid when |dialog_| is non-null.
+  content::JavaScriptMessageType message_type_ =
+      content::JavaScriptMessageType::JAVASCRIPT_MESSAGE_TYPE_ALERT;
+
   // The callback provided for when the dialog is closed. Usually the dialog
   // itself calls it, but in the cases where the dialog is closed not by the
   // user's input but by a call to |CloseDialog|, this class will call it.