Refactor GN source expansions.

tools/gn/source_file.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/source_file.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_dir.h"
 
 SourceFile::SourceFile() {
 }
 
 SourceFile::SourceFile(const base::StringPiece& p)
     : value_(p.data(), p.size()) {
   DCHECK(!value_.empty());
   DCHECK(value_[0] == '/');
   DCHECK(!EndsWithSlash(value_));
 }
 
+SourceFile::SourceFile(SwapIn, std::string* value) {
+  value_.swap(*value);
+  DCHECK(!value_.empty());
+  DCHECK(value_[0] == '/');
+  DCHECK(!EndsWithSlash(value_));
+}
+
 SourceFile::~SourceFile() {
 }
 
 std::string SourceFile::GetName() const {
   if (is_null())
     return std::string();
 
   DCHECK(value_.find('/') != std::string::npos);
   size_t last_slash = value_.rfind('/');
   return std::string(&value_[last_slash + 1],
@@ skipping 21 matching lines @@
     } else {
       converted.assign(value_);
     }
     return base::FilePath(UTF8ToFilePath(converted));
   }
 
   converted.assign(&value_[2], value_.size() - 2);
   return source_root.Append(UTF8ToFilePath(converted))
       .NormalizePathSeparatorsTo('/');
 }