Align base::hash_map with C++11.

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 <algorithm>
 #include <string>
 
@@ skipping 72 matching lines @@
  private:
   friend class SourceDir;
 
   std::string value_;
 
   // Copy & assign supported.
 };
 
 namespace BASE_HASH_NAMESPACE {
 
-#if defined(COMPILER_GCC)
 template<> struct hash<SourceFile> {
   std::size_t operator()(const SourceFile& v) const {
     hash<std::string> h;
     return h(v.value());
   }
 };
-#elif defined(COMPILER_MSVC)
-inline size_t hash_value(const SourceFile& v) {
-  return hash_value(v.value());
-}
-#endif  // COMPILER...
 
 }  // namespace BASE_HASH_NAMESPACE
 
 inline void swap(SourceFile& lhs, SourceFile& rhs) {
   lhs.swap(rhs);
 }
 
 #endif  // TOOLS_GN_SOURCE_FILE_H_