Add support for threadsafe completion callback factory.

ppapi/proxy/ppb_url_loader_proxy.h

Index: ppapi/proxy/ppb_url_loader_proxy.h
diff --git a/ppapi/proxy/ppb_url_loader_proxy.h b/ppapi/proxy/ppb_url_loader_proxy.h
index 7dea1e825e94218b1f557840584e37706d5dee08..5fd924d6f49b2996b0e6438fd3e828b66fbc5d06 100644
--- a/ppapi/proxy/ppb_url_loader_proxy.h
+++ b/ppapi/proxy/ppb_url_loader_proxy.h
@@ -14,7 +14,7 @@
 #include "ppapi/c/ppb_url_loader.h"
 #include "ppapi/c/trusted/ppb_url_loader_trusted.h"
 #include "ppapi/proxy/interface_proxy.h"
-#include "ppapi/proxy/proxy_non_thread_safe_ref_count.h"
+#include "ppapi/proxy/proxy_completion_callback_factory.h"
 #include "ppapi/shared_impl/host_resource.h"
 #include "ppapi/utility/completion_callback_factory.h"
 
@@ -86,8 +86,7 @@ class PPB_URLLoader_Proxy : public InterfaceProxy {
   // Handles callback for everything but reads.
   void OnCallback(int32_t result, const HostResource& resource);
 
-  pp::CompletionCallbackFactory<PPB_URLLoader_Proxy,
-                                ProxyNonThreadSafeRefCount> callback_factory_;
+  ProxyCompletionCallbackFactory<PPB_URLLoader_Proxy> callback_factory_;
 
   // Valid only in the host, this lazily-initialized pointer indicates the
   // URLLoaderTrusted interface.