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

Source/modules/serviceworkers/ServiceWorkerThread.cpp

Index: Source/modules/serviceworkers/ServiceWorkerThread.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorkerThread.cpp b/Source/modules/serviceworkers/ServiceWorkerThread.cpp
index 3a27a068169248e12f9213c16467cded55242b09..9ce0a57131fa10d49f0e60be3158703394351d6b 100644
--- a/Source/modules/serviceworkers/ServiceWorkerThread.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerThread.cpp
@@ -63,4 +63,13 @@ WebThreadSupportingGC& ServiceWorkerThread::backingThread()
     return *m_thread.get();
 }
 
+OwnPtr<WorkerV8Isolate>& ServiceWorkerThread::workerIsolate()
+{
+    if (!m_isolate) {
+        ASSERT(isCurrentThread());
+        m_isolate = WorkerV8Isolate::createDefault();
+    }
+    return m_isolate;
+}
+
 } // namespace blink