tools/gn: Add out-of-line copy ctors for complex classes.

tools/gn/token.cc

 // Copyright (c) 2013 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 "tools/gn/token.h"
 
 #include "base/logging.h"
 
 Token::Token() : type_(INVALID), value_() {
 }
 
 Token::Token(const Location& location,
              Type t,
              const base::StringPiece& v)
     : type_(t),
       value_(v),
       location_(location) {
 }
 
+Token::Token(const Token& other) = default;
+
 bool Token::IsIdentifierEqualTo(const char* v) const {
   return type_ == IDENTIFIER && value_ == v;
 }
 
 bool Token::IsStringEqualTo(const char* v) const {
   return type_ == STRING && value_ == v;
 }