Makes WebThreadSupportingGC constructible for an existing thread

Source/modules/compositorworker/CompositorWorkerManager.h

 // Copyright 2015 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 CompositorWorkerManager_h
 #define CompositorWorkerManager_h
 
 #include "modules/ModulesExport.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/ThreadingPrimitives.h"
 #include <v8.h>
 
 namespace blink {
 
 class V8IsolateInterruptor;
+class WebThread;
 class WebThreadSupportingGC;
 
 class MODULES_EXPORT CompositorWorkerManager final {
 public:
     static void initialize();
     static void shutdown();
 
     static CompositorWorkerManager* instance();
 
     // Returns the thread used for compositor workers. This creates a new thread if a
@@ skipping 12 matching lines @@
     void terminateV8Execution();
 
 private:
     friend class CompositorWorkerManagerTest;
 
     CompositorWorkerManager();
     ~CompositorWorkerManager();
 
     Mutex m_mutex;
     OwnPtr<WebThreadSupportingGC> m_thread;
+    OwnPtr<WebThread> m_platformThread;
     int m_workerCount = 0;
     v8::Isolate* m_isolate = nullptr;
 };
 
 } // namespace blink
 
 #endif // CompositorWorkerManager_h