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 ba4ffe788a48ec10470c5417a862aa172e9c6f0a..b16468ded1afe4617fe05d5c7128e0ea33888e1a 100644
--- a/base/strings/string16.h
+++ b/base/strings/string16.h
@@ -27,6 +27,8 @@
 // of carrying UTF-16-encoded data.
 
 #include <stdio.h>
+
+#include <functional>
 #include <string>
 
 #include "base/base_export.h"
@@ -180,6 +182,20 @@ 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 for consistency with WCHAR_T_IS_UTF16
+// platforms, where base::string16 is 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_