Make mojom-generated C++ string constants really constant.

mojo/public/tools/bindings/generators/cpp_templates/module-shared.h.tmpl

 // Copyright 2016 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.
 
 {%- set header_guard = "%s_SHARED_H_"|format(
         module.path|upper|replace("/","_")|replace(".","_")|
             replace("-", "_")) %}
 
 {%- macro mojom_type_traits(kind) %}
 template <>
 struct MojomTypeTraits<{{kind|get_qualified_name_for_kind}}DataView> {
   using Data = {{kind|get_qualified_name_for_kind(internal=True)}};
 {%-  if kind|is_union_kind %}
   using DataAsArrayElement = Data;
-  static const MojomTypeCategory category = MojomTypeCategory::UNION;
+  static constexpr MojomTypeCategory category = MojomTypeCategory::UNION;
 {%-  else %}
   using DataAsArrayElement = Pointer<Data>;
-  static const MojomTypeCategory category = MojomTypeCategory::STRUCT;
+  static constexpr MojomTypeCategory category = MojomTypeCategory::STRUCT;
 {%-  endif %}
 };
 {%- endmacro %}
 
 {%- macro namespace_begin() %}
 {%-   for namespace in namespaces_as_array %}
 namespace {{namespace}} {
 {%-   endfor %}
 {%- endmacro %}
 
@@ skipping 174 matching lines @@
 {%- endfor %}
 
 {%- for union in unions %}
 {%    include "union_data_view_definition.tmpl" %}
 {%- endfor %}
 
 {{namespace_end()}}
 
 #endif  // {{header_guard}}