Fix assert on Mandoline shutdown.

mandoline/tab/frame.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 MANDOLINE_TAB_FRAME_H_
 #define MANDOLINE_TAB_FRAME_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 34 matching lines @@
   mojo::View* view() { return view_; }
 
   void Add(Frame* node);
   void Remove(Frame* node);
 
  private:
   // mojo::ViewObserver:
   void OnViewDestroying(mojo::View* view) override;
 
   FrameTree* const tree_;
-  mojo::View* const view_;
+  mojo::View* view_;
   Frame* parent_;
   ViewOwnership view_ownership_;
   mojo::ServiceProviderImpl exposed_services_;
   mojo::ServiceProviderPtr services_;
   std::vector<Frame*> children_;
 
   DISALLOW_COPY_AND_ASSIGN(Frame);
 };
 
 }  // namespace mandoline
 
 #endif  // MANDOLINE_TAB_FRAME_H_