Delete remaining masks dead code.

sky/engine/core/rendering/style/RenderStyleConstants.h

Index: sky/engine/core/rendering/style/RenderStyleConstants.h
diff --git a/sky/engine/core/rendering/style/RenderStyleConstants.h b/sky/engine/core/rendering/style/RenderStyleConstants.h
index 7c51802cb1ede0f23602d169f3a033262ede8982..0eb142fd9dce5fb7990094ddde7bd611efe69abc 100644
--- a/sky/engine/core/rendering/style/RenderStyleConstants.h
+++ b/sky/engine/core/rendering/style/RenderStyleConstants.h
@@ -88,8 +88,9 @@ enum EFillRepeat {
     RepeatFill, NoRepeatFill, RoundFill, SpaceFill
 };
 
+// FIXME(sky): Remove this enum.
 enum EFillLayerType {
-    BackgroundFillLayer, MaskFillLayer
+    BackgroundFillLayer
 };
 
 // CSS3 Background Values
@@ -98,9 +99,6 @@ enum EFillSizeType { Contain, Cover, SizeLength, SizeNone };
 // CSS3 Background Position
 enum BackgroundEdgeOrigin { TopEdge, RightEdge, BottomEdge, LeftEdge };
 
-// CSS Mask Source Types
-enum EMaskSourceType { MaskAlpha, MaskLuminance };
-
 // Deprecated Flexible Box Properties
 
 enum EBoxPack { Start, Center, End, Justify };