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

tools/gn/err.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/err.h"
 
 #include <stddef.h>
 
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
@@ skipping 126 matching lines @@
     : has_error_(true),
       message_(msg),
       help_text_(help_text) {
   if (value.origin()) {
     LocationRange range = value.origin()->GetRange();
     location_ = range.begin();
     ranges_.push_back(range);
   }
 }
 
+Err::Err(const Err& other) = default;
+
 Err::~Err() {
 }
 
 void Err::PrintToStdout() const {
   InternalPrintToStdout(false);
 }
 
 void Err::AppendSubErr(const Err& err) {
   sub_errs_.push_back(err);
 }
@@ skipping 27 matching lines @@
   }
 
   // Optional help text.
   if (!help_text_.empty())
     OutputString(help_text_ + "\n");
 
   // Sub errors.
   for (const auto& sub_err : sub_errs_)
     sub_err.InternalPrintToStdout(true);
 }