Make mojo::Callback<>::Runnable::Run non-const.

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- import "struct_macros.tmpl" as struct_macros %}
 
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 {%- set namespace_as_string = "%s"|format(namespace|replace(".","::")) %}
 
 {%- macro alloc_params(struct, serialization_context) %}
   bool success = true;
 {%-   for param in struct.packed.packed_fields_in_ordinal_order %}
@@ skipping 207 matching lines @@
       uint64_t request_id,
       bool is_sync,
       mojo::MessageReceiverWithStatus* responder,
       scoped_refptr<mojo::internal::MultiplexRouter> router)
       : request_id_(request_id),
         is_sync_(is_sync),
         responder_(responder),
         serialization_context_(std::move(router)) {
   }
 
-  void Run({{interface_macros.declare_params("in_", method.response_parameters)}}) const override;
+  void Run({{interface_macros.declare_params("in_", method.response_parameters)}}) override;
 
  private:
   uint64_t request_id_;
   bool is_sync_;
-  mutable mojo::MessageReceiverWithStatus* responder_;
+  mojo::MessageReceiverWithStatus* responder_;
   // TODO(yzshen): maybe I should use a ref to the original one?
-  mutable mojo::internal::SerializationContext serialization_context_;
+  mojo::internal::SerializationContext serialization_context_;
   DISALLOW_COPY_AND_ASSIGN({{class_name}}_{{method.name}}_ProxyToResponder);
 };
 void {{class_name}}_{{method.name}}_ProxyToResponder::Run(
-    {{interface_macros.declare_params("in_", method.response_parameters)}}) const {
+    {{interface_macros.declare_params("in_", method.response_parameters)}}) {
   {{struct_macros.get_serialized_size(response_params_struct, "in_%s",
                                       "&serialization_context_")}}
   mojo::internal::ResponseMessageBuilder builder(
       {{message_name}}, size, request_id_,
       is_sync_ ? mojo::internal::kMessageIsSync : 0);
   {{build_message(response_params_struct, "in_%s", params_description,
                   "&serialization_context_")}}
   bool ok = responder_->Accept(builder.message());
   ALLOW_UNUSED_LOCAL(ok);
   // TODO(darin): !ok returned here indicates a malformed message, and that may
@@ skipping 153 matching lines @@
     default:
       break;
   }
 
   // Unrecognized message.
   ReportValidationError(
       mojo::internal::VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD);
   return false;
 }
 {%- endif -%}