Turn WorkerLoaderProxy into a threadsafe, ref-counted object.

Source/core/loader/WorkerThreadableLoader.h

Index: Source/core/loader/WorkerThreadableLoader.h
diff --git a/Source/core/loader/WorkerThreadableLoader.h b/Source/core/loader/WorkerThreadableLoader.h
index d3666a0e113f5742292b1758b7a57f058e2d84a8..4dfdcab58894fb35135dcb93817397fb3f82866c 100644
--- a/Source/core/loader/WorkerThreadableLoader.h
+++ b/Source/core/loader/WorkerThreadableLoader.h
@@ -87,7 +87,7 @@ namespace blink {
         class MainThreadBridge final : public ThreadableLoaderClient {
         public:
             // All executed on the worker context's thread.
-            MainThreadBridge(PassRefPtr<ThreadableLoaderClientWrapper>, PassOwnPtr<ThreadableLoaderClient>, WorkerLoaderProxy&, const ResourceRequest&, const ThreadableLoaderOptions&, const ResourceLoaderOptions&, const String& outgoingReferrer);
+            MainThreadBridge(PassRefPtr<ThreadableLoaderClientWrapper>, PassOwnPtr<ThreadableLoaderClient>, PassRefPtr<WorkerLoaderProxy>, const ResourceRequest&, const ThreadableLoaderOptions&, const ResourceLoaderOptions&, const String& outgoingReferrer);
             void overrideTimeout(unsigned long timeoutMilliseconds);
             void cancel();
             void destroy();
@@ -122,7 +122,7 @@ namespace blink {
             RefPtr<ThreadableLoaderClientWrapper> m_workerClientWrapper;
 
             // Used on the worker context thread.
-            WorkerLoaderProxy& m_loaderProxy;
+            RefPtr<WorkerLoaderProxy> m_loaderProxy;
         };
 
         WorkerThreadableLoader(WorkerGlobalScope&, PassRefPtr<ThreadableLoaderClientWrapper>, PassOwnPtr<ThreadableLoaderClient>, const ResourceRequest&, const ThreadableLoaderOptions&, const ResourceLoaderOptions&);