Remove more oilpan.

sky/engine/core/dom/DocumentTest.cpp

Index: sky/engine/core/dom/DocumentTest.cpp
diff --git a/sky/engine/core/dom/DocumentTest.cpp b/sky/engine/core/dom/DocumentTest.cpp
index 8bd8b16a0caaa5f74dd5917a97b8e284339433ca..258878b1af1591a4161e8208b568c3ed0b36aa13 100644
--- a/sky/engine/core/dom/DocumentTest.cpp
+++ b/sky/engine/core/dom/DocumentTest.cpp
@@ -56,10 +56,7 @@ void DocumentTest::SetUp()
     m_dummyPageHolder = DummyPageHolder::create(IntSize(800, 600));
 }
 
-class MockDocumentVisibilityObserver
-    : public DummyBase<MockDocumentVisibilityObserver>
-    , public DocumentVisibilityObserver {
-    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(MockDocumentVisibilityObserver);
+class MockDocumentVisibilityObserver : public DocumentVisibilityObserver {
 public:
     static PassOwnPtr<MockDocumentVisibilityObserver> create(Document& document)
     {