Fix data race in blink::WorkerMessagingProxy::reportException

Source/core/workers/WorkerThread.h

Index: Source/core/workers/WorkerThread.h
diff --git a/Source/core/workers/WorkerThread.h b/Source/core/workers/WorkerThread.h
index a4c57754c1ee1b7f7a9f82ae2900ec319c0d9df3..1685abcf7afdde37cbd03a3229a6ca17e5e57cee 100644
--- a/Source/core/workers/WorkerThread.h
+++ b/Source/core/workers/WorkerThread.h
@@ -106,7 +106,8 @@ public:
     void willEnterNestedLoop();
     void didLeaveNestedLoop();
 
-    WorkerGlobalScope* workerGlobalScope() const { return m_workerGlobalScope.get(); }
+    // Can be called only on the worker thread, WorkerGlobalScope is not thread safe.
+    WorkerGlobalScope* workerGlobalScope();
 
     // Returns true once one of the terminate* methods is called.
     bool terminated();