Add support for threadsafe completion callback factory.

ppapi/proxy/ppb_file_system_proxy.h

Index: ppapi/proxy/ppb_file_system_proxy.h
diff --git a/ppapi/proxy/ppb_file_system_proxy.h b/ppapi/proxy/ppb_file_system_proxy.h
index 86eb48e01606352dc00eae8fca5f4370a17cc00a..68197e7c112be6ecb836e070fc71fd9001477199 100644
--- a/ppapi/proxy/ppb_file_system_proxy.h
+++ b/ppapi/proxy/ppb_file_system_proxy.h
@@ -13,7 +13,7 @@
 #include "ppapi/c/pp_time.h"
 #include "ppapi/c/ppb_file_system.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/utility/completion_callback_factory.h"
 
 namespace ppapi {
@@ -51,8 +51,7 @@ class PPB_FileSystem_Proxy : public InterfaceProxy {
   void OpenCompleteInHost(int32_t result,
                           const ppapi::HostResource& host_resource);
 
-  pp::CompletionCallbackFactory<PPB_FileSystem_Proxy,
-                                ProxyNonThreadSafeRefCount> callback_factory_;
+  ProxyCompletionCallbackFactory<PPB_FileSystem_Proxy> callback_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PPB_FileSystem_Proxy);
 };