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

ash/frame/custom_frame_view_ash.h

 // Copyright (c) 2012 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 ASH_FRAME_CUSTOM_FRAME_VIEW_ASH_H_
 #define ASH_FRAME_CUSTOM_FRAME_VIEW_ASH_H_
 
 #include "ash/ash_export.h"
 #include "base/memory/scoped_ptr.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/views/window/non_client_view.h"
 
 namespace ash {
 class FrameBorderHitTestController;
+class FrameCaptionButton;
 class FrameCaptionButtonContainerView;
 class ImmersiveFullscreenController;
 }
 namespace views {
 class Widget;
 }
 
 namespace ash {
 
 // A NonClientFrameView used for packaged apps, dialogs and other non-browser
@@ skipping 14 matching lines @@
   // and hides |header_view_| in immersive fullscreen.
   // CustomFrameViewAsh does not take ownership of
   // |immersive_fullscreen_controller|.
   void InitImmersiveFullscreenControllerForView(
       ImmersiveFullscreenController* immersive_fullscreen_controller);
 
   // Sets the active and inactive frame colors. Note the inactive frame color
   // will have some transparency added when the frame is drawn.
   void SetFrameColors(SkColor active_frame_color, SkColor inactive_frame_color);
 
+  // Sets the left caption button.
+  void SetLeftCaptionButton(FrameCaptionButton* left_caption_button);
+
   // views::NonClientFrameView:
   gfx::Rect GetBoundsForClientView() const override;
   gfx::Rect GetWindowBoundsForClientBounds(
       const gfx::Rect& client_bounds) const override;
   int NonClientHitTest(const gfx::Point& point) override;
   void GetWindowMask(const gfx::Size& size, gfx::Path* window_mask) override;
   void ResetWindowControls() override;
   void UpdateWindowIcon() override;
   void UpdateWindowTitle() override;
   void SizeConstraintsChanged() override;
@@ skipping 35 matching lines @@
 
   // Updates the hittest bounds overrides based on the window state type.
   scoped_ptr<FrameBorderHitTestController> frame_border_hit_test_controller_;
 
   DISALLOW_COPY_AND_ASSIGN(CustomFrameViewAsh);
 };
 
 }  // namespace ash
 
 #endif  // ASH_FRAME_CUSTOM_FRAME_VIEW_ASH_H_