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

Source/core/workers/DedicatedWorkerThread.cpp

Index: Source/core/workers/DedicatedWorkerThread.cpp
diff --git a/Source/core/workers/DedicatedWorkerThread.cpp b/Source/core/workers/DedicatedWorkerThread.cpp
index 1567ddae3959d8e776c248f24f642644a0ffae52..da4a693d3db53dc60150bc4e02c0ff4f4466f73c 100644
--- a/Source/core/workers/DedicatedWorkerThread.cpp
+++ b/Source/core/workers/DedicatedWorkerThread.cpp
@@ -59,11 +59,11 @@ PassRefPtrWillBeRawPtr<WorkerGlobalScope> DedicatedWorkerThread::createWorkerGlo
     return DedicatedWorkerGlobalScope::create(this, startupData, m_timeOrigin);
 }
 
-WebThreadSupportingGC& DedicatedWorkerThread::backingThread()
+WebThread& DedicatedWorkerThread::backingThread()
 {
     if (!m_thread)
         m_thread = WebThreadSupportingGC::create("DedicatedWorker Thread");
-    return *m_thread.get();
+    return m_thread->platformThread();
 }
 
 void DedicatedWorkerThread::postInitialize()