Remove more oilpan.

sky/engine/core/editing/VisibleSelection.h

Index: sky/engine/core/editing/VisibleSelection.h
diff --git a/sky/engine/core/editing/VisibleSelection.h b/sky/engine/core/editing/VisibleSelection.h
index 9efcfa8d696af5682f84709f582c009df9b7c55d..3c2666da4349dca06e143ada930d7f19f8f998a2 100644
--- a/sky/engine/core/editing/VisibleSelection.h
+++ b/sky/engine/core/editing/VisibleSelection.h
@@ -117,7 +117,7 @@ public:
     // or end is moved to a different position.
     //
     // Objects implementing |ChangeObserver| interface must outlive the VisibleSelection object.
-    class ChangeObserver : public DummyBase<void> {
+    class ChangeObserver {
         WTF_MAKE_NONCOPYABLE(ChangeObserver);
     public:
         ChangeObserver();