Move const PortName data out of header file

mojo/edk/system/ports/name.cc

 // Copyright 2016 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/edk/system/ports/name.h"
 
 namespace mojo {
 namespace edk {
 namespace ports {
 
+extern const PortName kInvalidPortName = {0, 0};
+
+extern const NodeName kInvalidNodeName = {0, 0};
+
 std::ostream& operator<<(std::ostream& stream, const Name& name) {
   std::ios::fmtflags flags(stream.flags());
   stream << std::hex << std::uppercase << name.v1;
   if (name.v2 != 0)
     stream << '.' << name.v2;
   stream.flags(flags);
   return stream;
 }
 
 }  // namespace ports
 }  // namespace edk
 }  // namespace mojo