Oilpan: fix build after r177450.

Source/bindings/core/v8/ScriptPromisePropertyBase.h

Index: Source/bindings/core/v8/ScriptPromisePropertyBase.h
diff --git a/Source/bindings/core/v8/ScriptPromisePropertyBase.h b/Source/bindings/core/v8/ScriptPromisePropertyBase.h
index 2cecbdc05819ffc5871dc8193b6d368393ad60e4..2c199e8d0d4ee052217e4adc3406a07aefefe7b3 100644
--- a/Source/bindings/core/v8/ScriptPromisePropertyBase.h
+++ b/Source/bindings/core/v8/ScriptPromisePropertyBase.h
@@ -16,7 +16,7 @@ namespace WebCore {
 
 class ExecutionContext;
 
-class ScriptPromisePropertyBase : public ContextLifecycleObserver, public RefCounted<ScriptPromisePropertyBase> {
+class ScriptPromisePropertyBase : public RefCountedWillBeRefCountedGarbageCollected<ScriptPromisePropertyBase>, public ContextLifecycleObserver {
 public:
     virtual ~ScriptPromisePropertyBase();
 
@@ -35,6 +35,8 @@ public:
 
     ScriptPromise promise(DOMWrapperWorld&);
 
+    virtual void trace(Visitor*) { }
+
 protected:
     ScriptPromisePropertyBase(ExecutionContext*, Name);