PPAPI: Make CompletionCallbacks work right on background threads.

ppapi/shared_impl/ppb_message_loop_shared.cc

Index: ppapi/shared_impl/ppb_message_loop_shared.cc
diff --git a/ppapi/shared_impl/ppb_message_loop_shared.cc b/ppapi/shared_impl/ppb_message_loop_shared.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9c0c92d8b65779209a3ba712307835b6dfa6f339
--- /dev/null
+++ b/ppapi/shared_impl/ppb_message_loop_shared.cc
@@ -0,0 +1,20 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ppapi/shared_impl/ppb_message_loop_shared.h"
+
+namespace ppapi {
+
+MessageLoopShared::MessageLoopShared(PP_Instance instance)
+    : Resource(OBJECT_IS_PROXY, instance) {
+}
+
+MessageLoopShared::MessageLoopShared(ForMainThread)
+    : Resource(Resource::Untracked()) {
+}
+
+MessageLoopShared::~MessageLoopShared() {
+}
+
+}  // namespace ppapi