[MD Feedback] Add placeholder feedback entry point.

chrome/browser/ui/webui/md_feedback/md_feedback_dialog_controller.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_UI_WEBUI_MD_FEEDBACK_MD_FEEDBACK_DIALOG_CONTROLLER_H_
+#define CHROME_BROWSER_UI_WEBUI_MD_FEEDBACK_MD_FEEDBACK_DIALOG_CONTROLLER_H_
+
+#include "base/memory/singleton.h"
+#include "content/public/browser/browser_context.h"
+
+// Manages MD Feedback dialog creation, destruction, and showing a constrained
+// web dialog with the feedback contents. This controller is implemented as a
+// singleton.
+class MdFeedbackDialogController {
+ public:
+  static MdFeedbackDialogController* GetInstance();
+
+  // Show the feedback dialog. This WebUI dialog is not anchored to the browser
+  // window but tied to a profile.
+  void Show(content::BrowserContext* browser_context);
+
+ private:
+  friend struct base::DefaultSingletonTraits<MdFeedbackDialogController>;
+
+  MdFeedbackDialogController();
+  ~MdFeedbackDialogController() = default;
+
+  DISALLOW_COPY_AND_ASSIGN(MdFeedbackDialogController);
+};
+
+#endif  // CHROME_BROWSER_UI_WEBUI_MD_FEEDBACK_MD_FEEDBACK_DIALOG_CONTROLLER_H_