Make mojo_system static and mojo_system_impl a component, never use both

mojo/system/message_pipe.h

Index: mojo/system/message_pipe.h
diff --git a/mojo/system/message_pipe.h b/mojo/system/message_pipe.h
index de42c4492750d44e23ffa0e39c902df3f1ea8296..85cce0ae3d7d01cbc01c2cfe32df907d78fcccf1 100644
--- a/mojo/system/message_pipe.h
+++ b/mojo/system/message_pipe.h
@@ -24,9 +24,8 @@ class Channel;
 class Waiter;
 
 // |MessagePipe| is the secondary object implementing a message pipe (see the
-// explanatory comment in core_impl.cc). It is typically owned by the
-// dispatcher(s) corresponding to the local endpoints. This class is
-// thread-safe.
+// explanatory comment in core.cc). It is typically owned by the dispatcher(s)
+// corresponding to the local endpoints. This class is thread-safe.
 class MOJO_SYSTEM_IMPL_EXPORT MessagePipe :
     public base::RefCountedThreadSafe<MessagePipe> {
  public: