Oilpan: fix build after r187709.

Source/core/dom/ScopedWindowFocusAllowedIndicator.h

Index: Source/core/dom/ScopedWindowFocusAllowedIndicator.h
diff --git a/Source/core/dom/ScopedWindowFocusAllowedIndicator.h b/Source/core/dom/ScopedWindowFocusAllowedIndicator.h
index ea25492df4de1e822bf6a5754e99ba34129dc93a..4c2c81cf0aed1eeb5aa3f535af94049cf5b11276 100644
--- a/Source/core/dom/ScopedWindowFocusAllowedIndicator.h
+++ b/Source/core/dom/ScopedWindowFocusAllowedIndicator.h
@@ -38,6 +38,8 @@ private:
             if (executionContext())
                 executionContext()->consumeWindowFocus();
         }
+
+        void trace(Visitor*) { }
     };
 
     // In Oilpan, destructors are not allowed to touch other on-heap objects.