Oilpan: move WorkerGlobalScope to oilpan's heap.

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 7d6efc7e0b26cba91178b60b26e619b6bdb76e67..89a0353fd78af3f8ecce7a4acb8a4304b9a459f5 100644
--- a/Source/core/workers/WorkerThread.cpp
+++ b/Source/core/workers/WorkerThread.cpp
@@ -137,10 +137,13 @@ void WorkerThread::workerThread()
 
     ThreadIdentifier threadID = m_threadID;
 
+#if !ENABLE(OILPAN)
     ASSERT(m_workerGlobalScope->hasOneRef());
+#endif
 
     // The below assignment will destroy the context, which will in turn notify messaging proxy.
     // We cannot let any objects survive past thread exit, because no other thread will run GC or otherwise destroy them.
+    m_workerGlobalScope->detach();
     m_workerGlobalScope = nullptr;
 
     // Cleanup thread heap which causes all objects to be finalized.