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 433 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 is a region that defines the area that is visible
+  // (and matches the area that accepts input events). The region is specified
+  // in DIPs, so it must be scaled by the layer's |device_scale_factor|.

[danakj, 2014/07/11 16:08:22]

this comment seems out of date now

   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_