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

Source/core/loader/WorkerThreadableLoader.cpp

 /*
  * Copyright (C) 2009, 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 64 matching lines @@
     events.append(loaderDone.get());
 
     RefPtr<ThreadableLoaderClientWrapper> clientWrapper(ThreadableLoaderClientWrapper::create(&client));
     OwnPtr<WorkerLoaderClientBridgeSyncHelper> clientBridge(WorkerLoaderClientBridgeSyncHelper::create(client, loaderDone.release()));
 
     // This must be valid while loader is around.
     WorkerLoaderClientBridgeSyncHelper* clientBridgePtr = clientBridge.get();
 
     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();
         return;
     }
 
     clientBridgePtr->run();
 }
 
 void WorkerThreadableLoader::cancel()
@@ skipping 123 matching lines @@
 {
     m_clientBridge->didFailAccessControlCheck(error);
 }
 
 void WorkerThreadableLoader::MainThreadBridge::didFailRedirectCheck()
 {
     m_clientBridge->didFailRedirectCheck();
 }
 
 } // namespace WebCore