Fix the issue introduced by hooking window.onbeforeunload in WebDialogView which breaks WebDialogUI…

ui/web_dialogs/web_dialog_delegate.h

Index: ui/web_dialogs/web_dialog_delegate.h
diff --git a/ui/web_dialogs/web_dialog_delegate.h b/ui/web_dialogs/web_dialog_delegate.h
index 6a1d08d75ce38c3ef93899948adb8798347c6fa4..3a6ec4f1486b09e69bd183f831bae5b09411cfb8 100644
--- a/ui/web_dialogs/web_dialog_delegate.h
+++ b/ui/web_dialogs/web_dialog_delegate.h
@@ -79,6 +79,10 @@ class WEB_DIALOGS_EXPORT WebDialogDelegate {
   // registering it as a message handler in the WebUI object).
   virtual void OnDialogClosed(const std::string& json_retval) = 0;
 
+  // A callback to notify the delegate that the dialog is being closed in
+  // response to a "DialogClose" message from WebUI.
+  virtual void OnDialogCloseFromWebUI(const std::string& json_retval);
+
   // A callback to notify the delegate that the contents have gone
   // away. Only relevant if your dialog hosts code that calls
   // windows.close() and you've allowed that.  If the output parameter