Revert of Allow std::unordered_*.

base/strings/string16.h

Index: base/strings/string16.h
diff --git a/base/strings/string16.h b/base/strings/string16.h
index af44a5c5bf17a1ee6fd48bed9c794771b189b326..e47669c1b565e7cb79bfc275eb5965a9e506cbdf 100644
--- a/base/strings/string16.h
+++ b/base/strings/string16.h
@@ -29,8 +29,6 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
-
-#include <functional>
 #include <string>
 
 #include "base/base_export.h"
@@ -184,21 +182,6 @@
 extern template
 class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>;
 
-// Specialize std::hash for base::string16. Although the style guide forbids
-// this in general, it is necessary for consistency with WCHAR_T_IS_UTF16
-// platforms, where base::string16 is a type alias for std::wstring.
-namespace std {
-template<>
-struct hash<base::string16> {
-  std::size_t operator()(const base::string16& s) const {
-    std::size_t result = 0;
-    for (base::char16 c : s)
-      result = (result * 131) + c;
-    return result;
-  }
-};
-}  // namespace std
-
 #endif  // WCHAR_T_IS_UTF32
 
 #endif  // BASE_STRINGS_STRING16_H_