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

tools/gn/err.h

 // 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.
 
 #ifndef TOOLS_GN_ERR_H_
 #define TOOLS_GN_ERR_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 36 matching lines @@
   // Error at a given node.
   Err(const ParseNode* node,
       const std::string& msg,
       const std::string& help_text = std::string());
 
   // Error at a given value.
   Err(const Value& value,
       const std::string msg,
       const std::string& help_text = std::string());
 
+  Err(const Err& other);
+
   ~Err();
 
   bool has_error() const { return has_error_; }
   const Location& location() const { return location_; }
   const std::string& message() const { return message_; }
   const std::string& help_text() const { return help_text_; }
 
   void AppendRange(const LocationRange& range) { ranges_.push_back(range); }
   const RangeList& ranges() const { return ranges_; }
 
   void AppendSubErr(const Err& err);
 
   void PrintToStdout() const;
 
  private:
   void InternalPrintToStdout(bool is_sub_err) const;
 
   bool has_error_;
   Location location_;
 
   std::vector<LocationRange> ranges_;
 
   std::string message_;
   std::string help_text_;
 
   std::vector<Err> sub_errs_;
 };
 
 #endif  // TOOLS_GN_ERR_H_