Moves couple of WmShell methods to standalone functions

ash/mus/bridge/wm_shell_mus.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 ASH_MUS_BRIDGE_WM_SHELL_MUS_H_
 #define ASH_MUS_BRIDGE_WM_SHELL_MUS_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 44 matching lines @@
   }
 
   aura::WindowTreeClient* window_tree_client();
 
   WindowManager* window_manager() { return window_manager_; }
 
   // WmShell:
   void Shutdown() override;
   bool IsRunningInMash() const override;
   Config GetConfig() const override;
-  WmWindow* GetFocusedWindow() override;
-  WmWindow* GetActiveWindow() override;
-  WmWindow* GetCaptureWindow() override;
   WmWindow* GetPrimaryRootWindow() override;
   WmWindow* GetRootWindowForDisplayId(int64_t display_id) override;
   const display::ManagedDisplayInfo& GetDisplayInfo(
       int64_t display_id) const override;
   bool IsActiveDisplayId(int64_t display_id) const override;
   display::Display GetFirstDisplay() const override;
   bool IsInUnifiedMode() const override;
   bool IsInUnifiedModeIgnoreMirroring() const override;
   void SetDisplayWorkAreaInsets(WmWindow* window,
                                 const gfx::Insets& insets) override;
@@ skipping 69 matching lines @@
 
   std::unique_ptr<SessionStateDelegate> session_state_delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(WmShellMus);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_BRIDGE_WM_SHELL_MUS_H_