Add --use-more-webui runtime flag to toggle WebUI replacements for native dialogs.

chrome/browser/ui/webui/html_dialog_ui.cc

Index: chrome/browser/ui/webui/html_dialog_ui.cc
diff --git a/chrome/browser/ui/webui/html_dialog_ui.cc b/chrome/browser/ui/webui/html_dialog_ui.cc
index fd3341a4bbe187646a511a3a239a7d6a266e4bbf..9f01d1582ec1cc008bf7af2dac8e0ce505222a2d 100644
--- a/chrome/browser/ui/webui/html_dialog_ui.cc
+++ b/chrome/browser/ui/webui/html_dialog_ui.cc
@@ -29,6 +29,10 @@ HtmlDialogUI::~HtmlDialogUI() {
   // and the HTML dialogs won't swap WebUIs anyway since they don't navigate.
 }
 
+void HtmlDialogUI::CloseDialog(const base::ListValue* args) {
+  OnDialogClosed(args);
+}
+
 // static
 PropertyAccessor<HtmlDialogUIDelegate*>& HtmlDialogUI::GetPropertyAccessor() {
   return g_html_dialog_ui_property_accessor.Get();
@@ -67,8 +71,8 @@ void HtmlDialogUI::OnDialogClosed(const ListValue* args) {
       tab_contents()->property_bag());
   if (delegate) {
     std::string json_retval;
-    if (!args->GetString(0, &json_retval))
-      NOTREACHED() << "Could not read JSON arguments";
+    if (args && !args->empty() && !args->GetString(0, &json_retval))
+      NOTREACHED() << "Could not read JSON argument";
 
     (*delegate)->OnDialogClosed(json_retval);
   }