compositor-worker: Get the thread to run compositor-workers from the Platform.

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..584b4f542c589307493897f185573d850e8f7168 100644
--- a/Source/modules/serviceworkers/ServiceWorkerThread.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerThread.cpp
@@ -56,11 +56,11 @@ PassRefPtrWillBeRawPtr<WorkerGlobalScope> ServiceWorkerThread::createWorkerGloba
     return ServiceWorkerGlobalScope::create(this, startupData);
 }
 
-WebThreadSupportingGC& ServiceWorkerThread::backingThread()
+WebThread& ServiceWorkerThread::backingThread()
 {
     if (!m_thread)
         m_thread = WebThreadSupportingGC::create("ServiceWorker Thread");
-    return *m_thread.get();
+    return m_thread->platformThread();
 }
 
 } // namespace blink