exo: Add "back" button to shell surface decorations.

components/exo/shell_surface.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 COMPONENTS_EXO_SHELL_SURFACE_H_
 #define COMPONENTS_EXO_SHELL_SURFACE_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string16.h"
 #include "components/exo/surface_delegate.h"
 #include "components/exo/surface_observer.h"
+#include "ui/views/controls/button/button.h"
 #include "ui/views/widget/widget_delegate.h"
 
 namespace base {
 namespace trace_event {
 class TracedValue;
 }
 }
 
 namespace exo {
 class Surface;
@@ skipping 43 matching lines @@
   Surface* surface_;
   ui::WindowShowState show_state_;
   base::string16 title_;
 
   DISALLOW_COPY_AND_ASSIGN(ShellSurface);
 };
 
 }  // namespace exo
 
 #endif  // COMPONENTS_EXO_SHELL_SURFACE_H_