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

chrome/browser/ui/webui/chrome_web_ui.h

Index: chrome/browser/ui/webui/chrome_web_ui.h
diff --git a/chrome/browser/ui/webui/chrome_web_ui.h b/chrome/browser/ui/webui/chrome_web_ui.h
index 4b9ed852bfa0c23a65cb913a95c6dfb196ab8486..85ec192c1f16b261897eae9a2e3954574f32d206 100644
--- a/chrome/browser/ui/webui/chrome_web_ui.h
+++ b/chrome/browser/ui/webui/chrome_web_ui.h
@@ -24,6 +24,12 @@ class ChromeWebUI : public WebUI {
     return force_bookmark_bar_visible_;
   }
 
+  // SetMoreWebUI and IsMoreWebUI update and return the state of a global
+  // setting that tracks whether to use available WebUI implementations of
+  // native dialogs.
+  static void SetMoreWebUI(bool more_webui);
+  static bool IsMoreWebUI();
+
  protected:
   void set_force_bookmark_bar_visible(bool value) {
     force_bookmark_bar_visible_ = value;