compositor-worker: Create a separate CompositorWorkerGlobalScope for compositor workers.

Source/core/workers/DedicatedWorkerThread.h

Index: Source/core/workers/DedicatedWorkerThread.h
diff --git a/Source/core/workers/DedicatedWorkerThread.h b/Source/core/workers/DedicatedWorkerThread.h
index 8544e8384cde5fe1752b8117d08a11a096dc74d2..9cd49d19ec662378911f0460bd5c8c15b9b7e310 100644
--- a/Source/core/workers/DedicatedWorkerThread.h
+++ b/Source/core/workers/DedicatedWorkerThread.h
@@ -49,6 +49,8 @@ protected:
     virtual PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtrWillBeRawPtr<WorkerThreadStartupData>) override;
     virtual void postInitialize() override;
 
+    double timeOrigin() const { return m_timeOrigin; }
+
 private:
     WorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;