Make mojo_system static and mojo_system_impl a component, never use both

mojo/system/entrypoints.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_ENTRYPOINTS_H
+#define MOJO_SYSTEM_ENTRYPOINTS_H
+
+namespace mojo {
+namespace system {
+
+class Core;
+
+namespace entrypoints {
+
+// Sets the instance of Core to be used by system functions.
+void SetCore(Core* core);
+// Gets the instance of Core to be used by system functions.
+Core* GetCore();
+
+}  // namespace entrypoints
+}  // namepace system
+}  // namespace mojo
+
+#endif  // MOJO_SYSTEM_ENTRYPOINTS_H