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

Source/modules/compositorworker/CompositorWorkerThread.h

Index: Source/modules/compositorworker/CompositorWorkerThread.h
diff --git a/Source/modules/compositorworker/CompositorWorkerThread.h b/Source/modules/compositorworker/CompositorWorkerThread.h
index 7c62b3b23fd2a1545b0c9e80fecffa164875f24f..77c970bd875594dce9130b2e8accb17d9badeb40 100644
--- a/Source/modules/compositorworker/CompositorWorkerThread.h
+++ b/Source/modules/compositorworker/CompositorWorkerThread.h
@@ -24,10 +24,17 @@ private:
     // WorkerThread:
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
     WebThreadSupportingGC& backingThread() override;
+    void didStartRunLoop() override { }
+    void didStopRunLoop() override { }
+    void initializeBackingThread() override;
+    void shutdownBackingThread() override;
+
+    PassOwnPtr<WorkerV8Isolate> initializeIsolate() override;
+
+    WebThreadSupportingGC* m_thread = nullptr;
 
     WorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;
-    OwnPtr<WebThreadSupportingGC> m_thread;
 };
 
 } // namespace blink