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

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

 {%- import "validation_macros.tmpl" as validation_macros %}
 {%- set class_name = struct.name ~ "_Data" %}
 
 // static
 bool {{class_name}}::Validate(
     const void* data,
     mojo::internal::ValidationContext* validation_context) {
   if (!data)
     return true;
 
   if (!ValidateStructHeaderAndClaimMemory(data, validation_context))
     return false;
 
   // NOTE: The memory backing |object| may be smaller than |sizeof(*object)| if
   // the message comes from an older version.
   const {{class_name}}* object = static_cast<const {{class_name}}*>(data);
 
-  static const struct {
+  static constexpr struct {
     uint32_t version;
     uint32_t num_bytes;
   } kVersionSizes[] = {
 {%- for version in struct.versions -%}
     { {{version.version}}, {{version.num_bytes}} }{% if not loop.last %}, {% endif -%}
 {%- endfor -%}
   };
 
   if (object->header_.version <=
           kVersionSizes[arraysize(kVersionSizes) - 1].version) {
@@ skipping 32 matching lines @@
     return true;
 {%-     endif %}
 {%-     set field_expr = "object->" ~ packed_field.field.name %}
 {{validation_macros.validate_field(packed_field.field, field_expr, struct.name, true)}}
 {%-   endif %}
 {%- endfor %}
 
   return true;
 }