Revert of compositor-worker: Share a thread and an isolate for compositor workers.

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 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
-    // thread doesn't already exist.
-    WebThreadSupportingGC& compositorWorkerThread();
-
-    // Attempts to initialize/shutdown a thread if necessary. Does nothing if the thread
-    // is already initialized, or if the thread has more than one active workers at the
-    // time of shutdown.
-    void initializeBackingThread();
-    void shutdownBackingThread();
-
-    v8::Isolate* initializeIsolate();
-    void willDestroyIsolate();
-    void destroyIsolate();
-    void terminateV8Execution();
-
-private:
-    friend class CompositorWorkerManagerTest;
-
-    CompositorWorkerManager();
-    ~CompositorWorkerManager();
-
-    Mutex m_mutex;
-    OwnPtr<WebThreadSupportingGC> m_thread;
-    int m_workerCount = 0;
-    v8::Isolate* m_isolate = nullptr;
-    OwnPtr<V8IsolateInterruptor> m_interruptor;
-};
-
-} // namespace blink
-
-#endif // CompositorWorkerManager_h