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

mojo/system/test_embedder.h

-// 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.
-
-#ifndef MOJO_SYSTEM_TEST_EMBEDDER_H_
-#define MOJO_SYSTEM_TEST_EMBEDDER_H_
-
-namespace mojo {
-namespace embedder {
-namespace test {
-
-// This shuts down the global, singleton instance. (Note: "Real" embedders are
-// not expected to ever shut down this instance. This |Shutdown()| function will
-// do more work to ensure that tests don't leak, etc.)
-// TODO(vtl): Figure out the library/component/DLL/export situation for test
-// embedder stuff. For now, it's linked directly into the unit test binary.
-void Shutdown();
-
-}  // namespace test
-}  // namespace embedder
-}  // namespace mojo
-
-#endif  // MOJO_SYSTEM_EMBEDDER_H_