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..b9804c9f5bfea4bea3216082326d57eabc16b2ca 100644
--- a/Source/modules/compositorworker/CompositorWorkerThread.h
+++ b/Source/modules/compositorworker/CompositorWorkerThread.h
@@ -11,23 +11,32 @@ namespace blink {
 
 class WorkerObjectProxy;
 
-class CompositorWorkerThread final : public WorkerThread {
+// This class is overridden in unit-tests.
+class CompositorWorkerThread : public WorkerThread {
 public:
     static PassRefPtr<CompositorWorkerThread> create(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
     virtual ~CompositorWorkerThread();
 
     WorkerObjectProxy& workerObjectProxy() const { return m_workerObjectProxy; }
 
-private:
+protected:
     CompositorWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
 
+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