Refactor GN source expansions.

tools/gn/source_file.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_SOURCE_FILE_H_
 #define TOOLS_GN_SOURCE_FILE_H_
 
 #include <string>
 
 #include "base/containers/hash_tables.h"
 #include "base/files/file_path.h"
 #include "base/logging.h"
 #include "base/strings/string_piece.h"
 
 class SourceDir;
 
 // Represents a file within the source tree. Always begins in a slash, never
 // ends in one.
 class SourceFile {
  public:
+  enum SwapIn { SWAP_IN };
+
   SourceFile();
 
   // Takes a known absolute source file. Always begins in a slash.
   explicit SourceFile(const base::StringPiece& p);
 
+  // Constructs from the given string by swapping in the contents of the given
+  // value. The value will be the empty string after this call.
+  explicit SourceFile(SwapIn, std::string* value);

[scottmg, 2014/08/05 22:30:32]

no explicit necessary

+
   ~SourceFile();
 
   bool is_null() const { return value_.empty(); }
   const std::string& value() const { return value_; }
 
   // Returns everything after the last slash.
   std::string GetName() const;
   SourceDir GetDir() const;
 
   // Resolves this source file relative to some given source root. Returns
@@ skipping 52 matching lines @@
 };
 #elif defined(COMPILER_MSVC)
 inline size_t hash_value(const SourceFile& v) {
   return hash_value(v.value());
 }
 #endif  // COMPILER...
 
 }  // namespace BASE_HASH_NAMESPACE
 
 #endif  // TOOLS_GN_SOURCE_FILE_H_