chrome: Cleanup class/struct forward declarations

chrome/browser/ui/views/frame/browser_frame_mus.h

 // Copyright 2015 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_VIEWS_FRAME_BROWSER_FRAME_MUS_H_
 #define CHROME_BROWSER_UI_VIEWS_FRAME_BROWSER_FRAME_MUS_H_
 
 #include "base/macros.h"
 #include "chrome/browser/ui/views/frame/native_browser_frame.h"
 #include "ui/views/mus/native_widget_mus.h"
 
+class BrowserView;
+
 class BrowserFrameMus : public NativeBrowserFrame,
                         public views::NativeWidgetMus {
  public:
   BrowserFrameMus(BrowserFrame* browser_frame, BrowserView* browser_view);
   ~BrowserFrameMus() override;
 
   ui::Window* mus_window();
 
  private:
   // Overridden from NativeBrowserFrame:
@@ skipping 11 matching lines @@
 
   // Overriden from NativeWidgetMus:
   void UpdateClientArea() override;
 
   BrowserView* browser_view_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserFrameMus);
 };
 
 #endif  // CHROME_BROWSER_UI_VIEWS_FRAME_BROWSER_FRAME_MUS_H_