Allow Mojo structs as map keys

mojo/public/cpp/bindings/string.h

 // Copyright 2014 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_STRING_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRING_H_
 
 #include <stddef.h>
 
+#include <functional>
 #include <string>
 
 #include "base/logging.h"
 #include "mojo/public/cpp/bindings/lib/array_internal.h"
 #include "mojo/public/cpp/bindings/type_converter.h"
 
 namespace mojo {
 
 // A UTF-8 encoded character string that can be null. Provides functions that
 // are similar to std::string, along with access to the underlying std::string
@@ skipping 166 matching lines @@
 };
 
 template <>
 struct TypeConverter<String, const char*> {
   // |input| may be null, in which case a null String will be returned.
   static String Convert(const char* input) { return String(input); }
 };
 
 }  // namespace mojo
 
+namespace std {
+
+template <>
+struct hash<mojo::String> {
+  size_t operator()(const mojo::String& value) const {
+    // TODO(tibell): Is |value| nullable?

[yzshen1, 2016/09/20 23:44:40]

value.get() won't fail even if |value| is null.

[tibell, 2016/09/21 07:10:53]

Done.

+    return hash<std::string>()(value.get());
+  }
+};
+
+}  // namespace std
+
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_H_