Remove more oilpan.

sky/engine/core/css/CSSCanvasValue.h

Index: sky/engine/core/css/CSSCanvasValue.h
diff --git a/sky/engine/core/css/CSSCanvasValue.h b/sky/engine/core/css/CSSCanvasValue.h
index c2c16a00e788e4073a8131672c48553354c65907..ca8ad606ee4fc12ac201b3f031bfced5bb16c594 100644
--- a/sky/engine/core/css/CSSCanvasValue.h
+++ b/sky/engine/core/css/CSSCanvasValue.h
@@ -63,8 +63,7 @@ private:
 
     // NOTE: We put the CanvasObserver in a member instead of inheriting from it
     // to avoid adding a vptr to CSSCanvasValue.
-    class CanvasObserverProxy final : public DummyBase<CanvasObserverProxy>, public CanvasObserver {
-        WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(CanvasObserverProxy);
+    class CanvasObserverProxy final : public CanvasObserver {
     public:
         explicit CanvasObserverProxy(CSSCanvasValue* ownerValue) : m_ownerValue(ownerValue) { }