Converts MaximizeModeWindowManager to use ash/common types

ash/common/wm_window.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_COMMON_WM_WINDOW_H_
 #define ASH_COMMON_WM_WINDOW_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 255 matching lines @@
 
   // Makes the hit region for children slightly larger for easier resizing.
   virtual void SetChildrenUseExtendedHitRegion() = 0;
 
   // Sets whether descendants of this should not be moved to a different
   // container. This is used by SetBoundsInScreen().
   virtual void SetDescendantsStayInSameRootWindow(bool value) = 0;
 
   virtual void AddObserver(WmWindowObserver* observer) = 0;
   virtual void RemoveObserver(WmWindowObserver* observer) = 0;
+  virtual bool HasObserver(const WmWindowObserver* observer) const = 0;
 
  protected:
   virtual ~WmWindow() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_WINDOW_H_