Add CanCloseDialog to WebDialogDelegate to allow blocking closing of dialog if needed.

ui/web_dialogs/web_dialog_delegate.cc

Index: ui/web_dialogs/web_dialog_delegate.cc
diff --git a/ui/web_dialogs/web_dialog_delegate.cc b/ui/web_dialogs/web_dialog_delegate.cc
index ed414101abe05a217ee5a3aa99f378992a1bbec6..d3688e04cbe27d7a0a6304ae0c2812d62bd66e90 100644
--- a/ui/web_dialogs/web_dialog_delegate.cc
+++ b/ui/web_dialogs/web_dialog_delegate.cc
@@ -14,6 +14,10 @@ void WebDialogDelegate::GetMinimumDialogSize(gfx::Size* size) const {
   GetDialogSize(size);
 }
 
+bool WebDialogDelegate::CanCloseDialog() const {
+  return true;
+}
+
 void WebDialogDelegate::OnDialogCloseFromWebUI(
     const std::string& json_retval) {
   OnDialogClosed(json_retval);