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

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 class {{interface.name}}Proxy;
 
 template <typename ImplRefTraits>
 class {{interface.name}}Stub;
 
 class {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
 class {{interface.name}}ResponseValidator;
 {%- endif %}
 
 class {{export_attribute}} {{interface.name}}
     : public {{interface.name}}InterfaceBase {
  public:
   static const char Name_[];
-  static const uint32_t Version_ = {{interface.version}};
-  static const bool PassesAssociatedKinds_ = {% if interface|passes_associated_kinds %}true{% else %}false{% endif %};
-  static const bool HasSyncMethods_ = {% if interface|has_sync_methods %}true{% else %}false{% endif %};
+  static constexpr uint32_t Version_ = {{interface.version}};
+  static constexpr bool PassesAssociatedKinds_ = {% if interface|passes_associated_kinds %}true{% else %}false{% endif %};
+  static constexpr bool HasSyncMethods_ = {% if interface|has_sync_methods %}true{% else %}false{% endif %};
 
   using Proxy_ = {{interface.name}}Proxy;
 
   template <typename ImplRefTraits>
   using Stub_ = {{interface.name}}Stub<ImplRefTraits>;
 
   using RequestValidator_ = {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
   using ResponseValidator_ = {{interface.name}}ResponseValidator;
 {%- else %}
   using ResponseValidator_ = mojo::PassThroughFilter;
 {%- endif %}
 
 {#--- Metadata #}
   enum MethodMinVersions : uint32_t {
 {%- for method in interface.methods %}
     k{{method.name}}MinVersion = {{method.min_version|default(0, true)}},
 {%- endfor %}
   };
 
 {#--- Enums #}
 {%- for enum in interface.enums %}
   using {{enum.name}} = {{enum|get_name_for_kind(flatten_nested_kind=True)}};
 {%- endfor %}
 
 {#--- Constants #}
 {%- for constant in interface.constants %}
-{%-   if constant.kind|is_integral_kind %}
-  static const {{constant.kind|cpp_pod_type}} {{constant.name}} = {{constant|constant_value}};
-{%-   else %}
-  static const {{constant.kind|cpp_pod_type}} {{constant.name}};
-{%-   endif %}
+  static {{constant|format_constant_declaration(nested=True)}};
 {%- endfor %}
 
 {#--- Methods #}
   virtual ~{{interface.name}}() {}
 
 {%- for method in interface.methods %}
 {%    if method.response_parameters != None %}
 {%-     if method.sync %}
   // Sync method. This signature is used by the client side; the service side
   // should implement the signature with callback below.
   virtual bool {{method.name}}({{interface_macros.declare_sync_method_params("", method)}});
 {%-     endif %}
 
   using {{method.name}}Callback = {{interface_macros.declare_callback(method,
       for_blink, use_once_callback)}};
 {%-   endif %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method, use_once_callback)}}) = 0;
 {%- endfor %}
 };