Allow Mojo structs as map keys

mojo/public/cpp/bindings/string.h

Index: mojo/public/cpp/bindings/string.h
diff --git a/mojo/public/cpp/bindings/string.h b/mojo/public/cpp/bindings/string.h
index 7cfd713f66959bbaac1e17d92127039d11bf31c7..42f2dbc097ad0b0226788656273441a7c39a583d 100644
--- a/mojo/public/cpp/bindings/string.h
+++ b/mojo/public/cpp/bindings/string.h
@@ -7,6 +7,7 @@
 
 #include <stddef.h>
 
+#include <functional>
 #include <string>
 
 #include "base/logging.h"
@@ -193,4 +194,15 @@ struct TypeConverter<String, const char*> {
 
 }  // namespace mojo
 
+namespace std {
+
+template <>
+struct hash<mojo::String> {
+  size_t operator()(const mojo::String& value) const {
+    return hash<std::string>()(value.get());
+  }
+};
+
+}  // namespace std
+
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_H_