Mojo: Add support for constants to the IDL compiler.

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set class_name = struct.name ~ "_Data" %}
+
 // static
 {{class_name}}* {{class_name}}::New(mojo::Buffer* buf, mojo::Buffer::Destructor dtor) {
   return new (buf->Allocate(sizeof({{class_name}}), dtor)) {{class_name}}();
 }
 
 {{class_name}}::{{class_name}}() {
   _header_.num_bytes = sizeof(*this);
   _header_.num_fields = {{struct.packed.packed_fields|length}};
 }
 
@@ skipping 31 matching lines @@
 
 void {{class_name}}::EncodePointersAndHandles(
     std::vector<mojo::Handle>* handles) {
   {{ struct_macros.encodes(struct)|indent(2) }}
 }
 
 bool {{class_name}}::DecodePointersAndHandles(mojo::Message* message) {
   {{ struct_macros.decodes(struct)|indent(2) }}
   return true;
 }