Allow Mojo structs as map keys

mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl

 class {{export_attribute}} {{struct.name}} {
  public:
   using DataView = {{struct.name}}DataView;
   using Data_ = internal::{{struct.name}}_Data;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
   using {{enum.name}} = {{enum|get_name_for_kind(flatten_nested_kind=True)}};
 {%- endfor %}
 
@@ skipping 28 matching lines @@
   {{struct.name}}Ptr Clone() const;
 
   // Equals() is a template so it is only instantiated if it is used. Thus, the
   // bindings generator does not need to know whether Equals() or == operator
   // are available for members.
   template <typename T,
             typename std::enable_if<std::is_same<
                 T, {{struct.name}}>::value>::type* = nullptr>
   bool Equals(const T& other) const;
 
+{%- if struct|is_hashable %}
+  size_t Hash(size_t seed) const;
+{%- endif %}
+
 {%- set serialization_result_type = "mojo::WTFArray<uint8_t>"
         if for_blink else "mojo::Array<uint8_t>" %}
 
   template <typename UserType>
   static {{serialization_result_type}} Serialize(UserType* input) {
     return mojo::internal::StructSerializeImpl<
         {{struct.name}}::DataView, {{serialization_result_type}}>(input);
   }
 
   template <typename UserType>
@@ skipping 10 matching lines @@
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 
 {%- if struct|contains_move_only_members %}
  private:
   DISALLOW_COPY_AND_ASSIGN({{struct.name}});
 {%- endif %}
 };