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

Source/modules/compositorworker/CompositorWorkerManager.h

Index: Source/modules/compositorworker/CompositorWorkerManager.h
diff --git a/Source/modules/compositorworker/CompositorWorkerManager.h b/Source/modules/compositorworker/CompositorWorkerManager.h
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+// 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 "core/workers/WorkerV8Isolate.h"
+#include "wtf/OwnPtr.h"
+#include "wtf/PassOwnPtr.h"
+#include "wtf/ThreadingPrimitives.h"
+#include <v8.h>
+
+namespace blink {
+
+class WebThreadSupportingGC;
+
+class 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();
+
+    PassOwnPtr<WorkerV8Isolate> createIsolate();
+    void willDestroyIsolate();
+    void destroyIsolate();
+    void terminateIsolate();
+
+private:
+    CompositorWorkerManager();
+    ~CompositorWorkerManager();
+
+    Mutex m_mutex;
+    OwnPtr<WebThreadSupportingGC> m_thread;
+    int m_workerCount = 0;
+    OwnPtr<WorkerV8Isolate> m_isolate;
+    int m_isolateCount = 0;
+};
+
+} // namespace blink
+
+#endif // CompositorWorkerManager_h