Mojo: Move test_embedder.* to the mojo_system_impl component.

mojo/system/embedder/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_EMBEDDER_TEST_EMBEDDER_H_
 #define MOJO_SYSTEM_EMBEDDER_TEST_EMBEDDER_H_
 
+#include "mojo/system/system_impl_export.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();
+MOJO_SYSTEM_IMPL_EXPORT void Shutdown();
 
 }  // namespace test
 }  // namespace embedder
 }  // namespace mojo
 
 #endif  // MOJO_SYSTEM_EMBEDDER_EMBEDDER_H_