Determine mask UVs based on texture size

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 58dde9c4577aa05aca94dd2f73aab70af8e0f7d7..b499d6125f4d33206fa4c474cd0b449d5ac9870e 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -115,6 +115,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
   void SetMasksToBounds(bool masks_to_bounds);
   bool masks_to_bounds() const { return inputs_.masks_to_bounds; }
 
+  // Note that the mask layer is not scaled to fill the layer being masked.
   void SetMaskLayer(Layer* mask_layer);
   Layer* mask_layer() { return inputs_.mask_layer.get(); }
   const Layer* mask_layer() const { return inputs_.mask_layer.get(); }