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

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

Index: mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
index c437738107086ff02e89c67de9b330aa49d35b8d..8ed0f96ab97f051fd0452c1177a38ccea2d7070f 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
@@ -225,18 +225,18 @@ class {{class_name}}_{{method.name}}_ProxyToResponder
         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(