Scale the ALPHA_THRESHOLD filter's region based on the device scale factor.

ui/compositor/layer.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 UI_COMPOSITOR_LAYER_H_
 #define UI_COMPOSITOR_LAYER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 356 matching lines @@
       scoped_ptr<cc::Animation> animation) OVERRIDE;
   virtual void RemoveThreadedAnimation(int animation_id) OVERRIDE;
 
   // Creates a corresponding composited layer for |type_|.
   void CreateWebLayer();
 
   // Recomputes and sets to |cc_layer_|.
   void RecomputeDrawsContentAndUVRect();
   void RecomputePosition();
 
+  void RecomputeAlphaShape();

[Wez, 2014/06/08 00:55:54]

Add an explanatory comment?

[garykac, 2014/06/09 21:30:00]

Done.

+
   // Set all filters which got applied to the layer.
   void SetLayerFilters();
 
   // Set all filters which got applied to the layer background.
   void SetLayerBackgroundFilters();
 
   // Cleanup |cc_layer_| and replaces it with |new_layer|.
   void SwitchToLayer(scoped_refptr<cc::Layer> new_layer);
 
   // We cannot send animations to our cc_layer_ until we have been added to a
@@ skipping 41 matching lines @@
   // while attached to the main layer before the main layer is deleted.
   Layer* layer_mask_back_link_;
 
   // The zoom factor to scale the layer by.  Zooming is disabled when this is
   // set to 1.
   float zoom_;
 
   // Width of the border in pixels, where the scaling is blended.
   int zoom_inset_;
 
-  // Shape of the window.
+  // The alpha shape specifies the area that is visible (and matches the area
+  // that accepts input events).
+  // |raw_alpha_shape_| is the shape as specified and |alpha_shape_| is the
+  // shape after being adjusted by the |device_scale_factor_|.
+  scoped_ptr<SkRegion> raw_alpha_shape_;

[Wez, 2014/06/08 00:55:54]

nit: Would |dips_alpha_shape_| and |device_alpha_shape_| here be clearer;
otherwise you have end up having to explain what the "raw" shape is, as in your
comment. :)

[garykac, 2014/06/09 21:29:59]

I updated raw_ to dips_ and updated the comment.

   scoped_ptr<SkRegion> alpha_shape_;
 
   std::string name_;
 
   LayerDelegate* delegate_;
 
   LayerOwner* owner_;
 
   scoped_refptr<LayerAnimator> animator_;
 
@@ skipping 22 matching lines @@
   // The size of the frame or texture in DIP, set when SetShowDelegatedContent
   // or SetTextureMailbox was called.
   gfx::Size frame_size_in_dip_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_H_