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

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, params, message, serialization_context,
                        description) %}
   ({{serialization_context}})->handles.Swap(({{message}})->mutable_handles());
@@ skipping 24 matching lines @@
                         serialization_context) -%}
   {{struct_macros.serialize(struct, struct_display_name, input_pattern,
                             "params", "builder.buffer()",
                             serialization_context)}}
   ({{serialization_context}})->handles.Swap(
       builder.message()->mutable_handles());
 {%- endmacro %}
 
 {#--- Begin #}
 const char {{class_name}}::Name_[] = "{{namespace_as_string}}::{{class_name}}";
-const uint32_t {{class_name}}::Version_;
 
 {#--- Constants #}
 {%-  for constant in interface.constants %}
-{%-   if constant.kind|is_integral_kind %}
-const {{constant.kind|cpp_pod_type}} {{interface.name}}::{{constant.name}};
-{%-   else %}
-const {{constant.kind|cpp_pod_type}} {{interface.name}}::{{constant.name}} = {{constant|constant_value}};
+{%-   if constant.kind|is_string_kind %}
+const char {{interface.name}}::{{constant.name}}[] = {{constant|constant_value}};
 {%-   endif %}
 {%- endfor %}
 
 
 {%- for method in interface.methods %}
 {%-   if method.sync %}
 bool {{class_name}}::{{method.name}}({{interface_macros.declare_sync_method_params("", method)}}) {
   NOTREACHED();
   return false;
 }
@@ skipping 383 matching lines @@
       break;
   }
 
   // Unrecognized message.
   ReportValidationError(
       &validation_context,
       mojo::internal::VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD);
   return false;
 }
 {%- endif -%}