[ash-md] Improves smoothness with many windows in overview

ash/common/wm/overview/scoped_transform_overview_window.h

 // Copyright 2013 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_OVERVIEW_SCOPED_TRANSFORM_OVERVIEW_WINDOW_H_
 #define ASH_COMMON_WM_OVERVIEW_SCOPED_TRANSFORM_OVERVIEW_WINDOW_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 87 matching lines @@
 
   // Informs the ScopedTransformOverviewWindow that the window being watched was
   // destroyed. This resets the internal window pointer.
   void OnWindowDestroyed();
 
   // Prepares for overview mode by doing any necessary actions before entering.
   void PrepareForOverview();
 
   // Applies the |transform| to the overview window and all of its transient
   // children. With Material Design creates a mask layer with the bottom edge
-  // using rounded corners of |radius|.
+  // using rounded corners of |radius|. When |use_mask| is set, hides the
+  // original window header and uses rounded rectangle mask which may be
+  // resource-intensive. When |use_shape| is set and |use_mask| is not, uses
+  // SetAlphaShape to mask the header.
   void SetTransform(WmWindow* root_window,
                     const gfx::Transform& transform,
+                    bool use_mask,
+                    bool use_shape,
                     float radius);
 
   // Set's the opacity of the managed windows.
   void SetOpacity(float opacity);
 
   WmWindow* window() const { return window_; }
 
   // Closes the transient root of the window managed by |this|.
   void Close();
 
@@ skipping 50 matching lines @@
   float original_opacity_;
 
   base::WeakPtrFactory<ScopedTransformOverviewWindow> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedTransformOverviewWindow);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_OVERVIEW_SCOPED_TRANSFORM_OVERVIEW_WINDOW_H_