compositor-worker: Share a thread and an isolate 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 a26b28d67f3ea931b59c198d37e00ba76d455d64..4770cd4898e722ff5165a8e8695d4059732a6d1f 100644
--- a/Source/core/workers/DedicatedWorkerThread.h
+++ b/Source/core/workers/DedicatedWorkerThread.h
@@ -48,6 +48,7 @@ protected:
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
     void postInitialize() override;
     WebThreadSupportingGC& backingThread() override;
+    OwnPtr<WorkerV8Isolate>& workerIsolate() override;
 
 private:
     DedicatedWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
@@ -55,6 +56,7 @@ private:
     WorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;
     OwnPtr<WebThreadSupportingGC> m_thread;
+    OwnPtr<WorkerV8Isolate> m_isolate;
 };
 
 } // namespace blink