Oilpan: fix build after r187699.

Source/core/inspector/V8AsyncCallTracker.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8AsyncCallTracker_h
 #define V8AsyncCallTracker_h
 
 #include "bindings/core/v8/ScriptState.h"
 #include "core/inspector/InspectorDebuggerAgent.h"
 #include "platform/heap/Handle.h"
 #include "wtf/Forward.h"
 #include "wtf/HashMap.h"
 #include "wtf/Noncopyable.h"
 
 namespace blink {
 
 class InspectorDebuggerAgent;
 class ScriptState;
 
 class V8AsyncCallTracker final : public NoBaseWillBeGarbageCollectedFinalized<V8AsyncCallTracker>, public ScriptState::Observer, public InspectorDebuggerAgent::AsyncCallTrackingListener {
     WTF_MAKE_NONCOPYABLE(V8AsyncCallTracker);
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(V8AsyncCallTracker);
 public:
     explicit V8AsyncCallTracker(InspectorDebuggerAgent*);
     ~V8AsyncCallTracker();
     void trace(Visitor*);
 
     // InspectorDebuggerAgent::AsyncCallTrackingListener implementation:
     void asyncCallTrackingStateChanged(bool tracking) override;
     void resetAsyncCallChains() override;
 
     void didReceiveV8AsyncTaskEvent(ScriptState*, const String& eventType, const String& eventName, int id);
 
     // ScriptState::Observer implementation:
     void willDisposeScriptState(ScriptState*) override;
 
 private:
     void didEnqueueV8AsyncTask(ScriptState*, const String& eventName, int id);
     void willHandleV8AsyncTask(ScriptState*, const String& eventName, int id);
 
     class V8ContextAsyncCallChains;
     using V8ContextAsyncChainMap = WillBeHeapHashMap<ScriptState*, OwnPtrWillBeMember<V8ContextAsyncCallChains> >;
     V8ContextAsyncChainMap m_contextAsyncCallChainMap;
     RawPtrWillBeMember<InspectorDebuggerAgent> m_debuggerAgent;
 };
 
 } // namespace blink
 
 #endif // V8AsyncCallTracker_h