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 e47669c1b565e7cb79bfc275eb5965a9e506cbdf..af44a5c5bf17a1ee6fd48bed9c794771b189b326 100644
--- a/base/strings/string16.h
+++ b/base/strings/string16.h
@@ -29,6 +29,8 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
+
+#include <functional>
 #include <string>
 
 #include "base/base_export.h"
@@ -182,6 +184,21 @@ BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
 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_