workers: Move ownership of WebThread from WorkerThread

Source/core/workers/WorkerThread.cpp

Index: Source/core/workers/WorkerThread.cpp
diff --git a/Source/core/workers/WorkerThread.cpp b/Source/core/workers/WorkerThread.cpp
index f59a49182a66ff3dcd4b57653f081d209767d181..8679bc071b18915b966013409b89b160f95d1d22 100644
--- a/Source/core/workers/WorkerThread.cpp
+++ b/Source/core/workers/WorkerThread.cpp
@@ -265,6 +265,7 @@ WorkerThread::WorkerThread(const char* threadName, PassRefPtr<WorkerLoaderProxy>
     , m_isolate(nullptr)
     , m_shutdownEvent(adoptPtr(blink::Platform::current()->createWaitableEvent()))
     , m_terminationEvent(adoptPtr(blink::Platform::current()->createWaitableEvent()))
+    , m_thread(nullptr)
 {
     MutexLocker lock(threadSetMutex());
     workerThreads().add(this);
@@ -281,8 +282,11 @@ void WorkerThread::start()
 {
     if (m_thread)
         return;
-
-    m_thread = createWebThreadSupportingGC();
+    m_thread = m_startupData->m_thread;
+    if (!m_thread) {
+        m_ownThread = createWebThreadSupportingGC();
+        m_thread = m_ownThread.get();
+    }
     m_thread->postTask(FROM_HERE, new Task(threadSafeBind(&WorkerThread::initialize, AllowCrossThreadAccess(this))));
 }