[worklets] Split apart CompositorWorkerThread for sharing with AnimationWorkletThread.

third_party/WebKit/Source/modules/compositorworker/AbstractAnimationWorkletThread.h

+// Copyright 2016 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 AbstractAnimationWorkletThread_h
+#define AbstractAnimationWorkletThread_h
+
+#include "core/workers/WorkerLoaderProxy.h"
+#include "core/workers/WorkerThread.h"
+#include "modules/ModulesExport.h"
+#include <memory>
+
+namespace blink {
+
+class WorkerReportingProxy;
+
+// TODO(ikilpatrick): Remove this class up to AnimationWorkletThread once we no
+// longer have CompositorWorker.
+class MODULES_EXPORT AbstractAnimationWorkletThread : public WorkerThread {
+public:
+    ~AbstractAnimationWorkletThread() override;
+
+    WorkerBackingThread& workerBackingThread() override;
+    bool shouldAttachThreadDebugger() const override { return false; }
+
+    static void ensureSharedBackingThread();
+    static void createSharedBackingThreadForTest();
+
+    static void clearSharedBackingThread();
+
+protected:
+    AbstractAnimationWorkletThread(PassRefPtr<WorkerLoaderProxy>, WorkerReportingProxy&);
+
+    bool isOwningBackingThread() const override { return false; }
+};
+
+} // namespace blink
+
+#endif // AbstractAnimationWorkletThread_h