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

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 2b0d9138361dd46eabb0dbe199716c63c2256f3e..e30f37c202821116ab1336cb179263bf0f85fb67 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -374,6 +374,9 @@ class COMPOSITOR_EXPORT Layer
   void RecomputeDrawsContentAndUVRect();
   void RecomputePosition();
 
+  // Recomputes the AlphaShape mask based on the |device_scaling_factor|.
+  void RecomputeAlphaShape();
+
   // Set all filters which got applied to the layer.
   void SetLayerFilters();
 
@@ -435,7 +438,11 @@ class COMPOSITOR_EXPORT Layer
   // 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).
+  // |dips_alpha_shape_| is the device independent shape.
+  // |alpha_shape_| is the shape adjusted by the |device_scale_factor_|.
+  scoped_ptr<SkRegion> dips_alpha_shape_;
   scoped_ptr<SkRegion> alpha_shape_;
 
   std::string name_;