Mojo: Move system embedder API files to mojo/system/embedder.

mojo/system/test_embedder.cc

-// Copyright 2014 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 "mojo/system/test_embedder.h"
-
-#include "base/logging.h"
-#include "mojo/system/core_impl.h"
-
-namespace mojo {
-namespace embedder {
-namespace test {
-
-void Shutdown() {
-  system::CoreImpl* core_impl = static_cast<system::CoreImpl*>(Core::Get());
-  CHECK(core_impl);
-  Core::Reset();
-
-  // TODO(vtl): Check for leaks, etc.
-
-  delete core_impl;
-}
-
-}  // namespace test
-}  // namespace embedder
-}  // namespace mojo