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

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..6009d8e3691168b7133bc340780460c41551ba70 100644
--- a/Source/core/workers/SharedWorkerThread.cpp
+++ b/Source/core/workers/SharedWorkerThread.cpp
@@ -57,11 +57,11 @@ PassRefPtrWillBeRawPtr<WorkerGlobalScope> SharedWorkerThread::createWorkerGlobal
     return SharedWorkerGlobalScope::create(m_name, this, startupData);
 }
 
-WebThreadSupportingGC& SharedWorkerThread::backingThread()
+WebThread& SharedWorkerThread::backingThread()
 {
     if (!m_thread)
         m_thread = WebThreadSupportingGC::create("SharedWorker Thread");
-    return *m_thread.get();
+    return m_thread->platformThread();
 }
 
 } // namespace blink