Oilpan: fix build after r187699.

Source/core/inspector/InspectorDebuggerAgent.h

Index: Source/core/inspector/InspectorDebuggerAgent.h
diff --git a/Source/core/inspector/InspectorDebuggerAgent.h b/Source/core/inspector/InspectorDebuggerAgent.h
index 0ba1ddcb1d46d60901acb1de9d882d71fd37b558..64d5ba9eeaab7be9c987096c5bccbd4a6dd156ec 100644
--- a/Source/core/inspector/InspectorDebuggerAgent.h
+++ b/Source/core/inspector/InspectorDebuggerAgent.h
@@ -180,9 +180,10 @@ public:
     void didCompleteAsyncOperation(AsyncCallChain*);
     bool trackingAsyncCalls() const { return m_maxAsyncCallStackDepth; }
 
-    class AsyncCallTrackingListener {
+    class AsyncCallTrackingListener : public WillBeGarbageCollectedMixin {
     public:
         virtual ~AsyncCallTrackingListener() { }
+        virtual void trace(Visitor*) { }
         virtual void asyncCallTrackingStateChanged(bool tracking) = 0;
         virtual void resetAsyncCallChains() = 0;
     };
@@ -291,10 +292,10 @@ private:
     RefPtrWillBeMember<AsyncCallChain> m_currentAsyncCallChain;
     unsigned m_nestedAsyncCallCount;
     bool m_performingAsyncStepIn;
-    WillBeHeapVector<AsyncCallTrackingListener*> m_asyncCallTrackingListeners;
+    WillBeHeapVector<RawPtrWillBeMember<AsyncCallTrackingListener>> m_asyncCallTrackingListeners;
 };
 
 } // namespace blink
 
 
-#endif // !defined(InspectorDebuggerAgent_h)
+#endif // InspectorDebuggerAgent_h