Mojom: Add 'default' keyword for initializing structs to non-null value.

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

-{%- macro get_default(field) -%}
-{%-   if field.default -%}
-{{field.default|expression_to_text}}
-{%-   endif -%}
-{%- endmacro -%}
-
 // static
 {{struct.name}}Ptr {{struct.name}}::New() {
   {{struct.name}}Ptr rv;
   mojo::internal::StructHelper<{{struct.name}}>::Initialize(&rv);
   return rv.Pass();
 }
 
 {{struct.name}}::{{struct.name}}()
 {%-for field in struct.fields %}
-    {% if loop.first %}:{% else %} {% endif %} {{field.name}}({{get_default(field)}}){% if not loop.last %},{% endif %}
+    {% if loop.first %}:{% else %} {% endif %} {{field.name}}({{field|default_value}}){% if not loop.last %},{% endif %}
 {%- endfor %} {
 }
 
 {{struct.name}}::~{{struct.name}}() {
 }