Align base::hash_map with C++11.

tools/gn/output_file.h

Index: tools/gn/output_file.h
diff --git a/tools/gn/output_file.h b/tools/gn/output_file.h
index eb6faf064f8c19cf32e1398a0878a34234e91633..3e0e99df47fc15049023f7c61287105648082060 100644
--- a/tools/gn/output_file.h
+++ b/tools/gn/output_file.h
@@ -47,18 +47,12 @@ class OutputFile {
 
 namespace BASE_HASH_NAMESPACE {
 
-#if defined(COMPILER_GCC)
 template<> struct hash<OutputFile> {
   std::size_t operator()(const OutputFile& v) const {
     hash<std::string> h;
     return h(v.value());
   }
 };
-#elif defined(COMPILER_MSVC)
-inline size_t hash_value(const OutputFile& v) {
-  return hash_value(v.value());
-}
-#endif  // COMPILER...
 
 }  // namespace BASE_HASH_NAMESPACE