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

Source/core/workers/SharedWorkerThread.cpp

Index: Source/core/workers/SharedWorkerThread.cpp
diff --git a/Source/core/workers/SharedWorkerThread.cpp b/Source/core/workers/SharedWorkerThread.cpp
index 576361da373a45a004afa3888df1cff46ad59773..f4a0ba11abc62d02c53f9b64dc6155c1166113e0 100644
--- a/Source/core/workers/SharedWorkerThread.cpp
+++ b/Source/core/workers/SharedWorkerThread.cpp
@@ -64,4 +64,13 @@ WebThreadSupportingGC& SharedWorkerThread::backingThread()
     return *m_thread.get();
 }
 
+OwnPtr<WorkerV8Isolate>& SharedWorkerThread::workerIsolate()
+{
+    if (!m_isolate) {
+        ASSERT(isCurrentThread());
+        m_isolate = WorkerV8Isolate::createDefault();
+    }
+    return m_isolate;
+}
+
 } // namespace blink