Make possible using pointer as a hash_table key when compiling using GCC.

base/hash_tables.h

 // Copyright (c) 2006-2008 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.
 //
 
 //
 // Deal with the differences between Microsoft and GNU implemenations
 // of hash_map. Allows all platforms to use |base::hash_map| and
 // |base::hash_set|.
-//  eg: 
+//  eg:
 //   base::hash_map<int> my_map;
 //   base::hash_set<int> my_set;
 //
 
 #ifndef BASE_HASH_TABLES_H__
 #define BASE_HASH_TABLES_H__
 
 #include "build/build_config.h"
 
 #if defined(COMPILER_MSVC)
@@ skipping 49 matching lines @@
   size_t operator()(const std::string& s) const {
     return std::tr1::hash<std::string>()(s);
   }
 };
 
 template<>
 struct hash<const std::string> {
   size_t operator()(const std::string& s) const {
     return std::tr1::hash<std::string>()(s);
   }
-};  
+};
+
+template<typename T>
+struct hash<T*> {
+  size_t operator()(T* s) const {
+    return std::tr1::hash<T*>()(s);
+  }
+};
+
+template<typename T>
+struct hash<const T*> {
+  size_t operator()(const T* s) const {
+    return std::tr1::hash<const T*>()(s);
+  }
+};
 
 }
 
 #endif
 
 #endif  // BASE_HASH_TABLES_H__