Avoid deadlock when attempting to GC during synchronous loading in a worker.

Source/core/loader/WorkerThreadableLoader.cpp

Index: Source/core/loader/WorkerThreadableLoader.cpp
diff --git a/Source/core/loader/WorkerThreadableLoader.cpp b/Source/core/loader/WorkerThreadableLoader.cpp
index 3f09f09fa1229b74d6814cc3d05f4b8f4c232463..b23c493e3c0758da064dd469877d7c04acc02273 100644
--- a/Source/core/loader/WorkerThreadableLoader.cpp
+++ b/Source/core/loader/WorkerThreadableLoader.cpp
@@ -82,6 +82,7 @@ void WorkerThreadableLoader::loadResourceSynchronously(WorkerGlobalScope* worker
 
     RefPtr<WorkerThreadableLoader> loader = WorkerThreadableLoader::create(workerGlobalScope, clientWrapper, clientBridge.release(), request, options);
 
+    ThreadState::SafePointScope scope(ThreadState::HeapPointersOnStack);
     blink::WebWaitableEvent* signalled = blink::Platform::current()->waitMultipleEvents(events);
     if (signalled == shutdownEvent) {
         loader->cancel();