Add support for enums within structs and interfaces to mojom.

mojo/public/bindings/generators/js_templates/struct_definition.tmpl

   function {{struct.name}}() {
 {%- for packed_field in struct.packed.packed_fields %}
     this.{{packed_field.field.name}} = {{packed_field.field|default_value}};
 {%- endfor %}
   }
 
+{#--- Enums #}
+{%- from "enum_definition.tmpl" import enum_def -%}
+{%  for enum in struct.enums %}
+{{  enum_def("%s.%s"|format(struct.name, enum.name), enum)}}
+{%  endfor %}
+
+{#--- Encoding and decoding #}
   {{struct.name}}.encodedSize = codec.kStructHeaderSize + {{struct.packed|payload_size}};
 
   {{struct.name}}.decode = function(decoder) {
     var packed;
     var val = new {{struct.name}}();
     var numberOfBytes = decoder.read32();
     var numberOfFields = decoder.read32();
 {%- for byte in struct.bytes %}
 {%-   if byte.packed_fields|length > 1 %}
     packed = decoder.read8();
@@ skipping 27 matching lines @@
 {%-   else %}
 {%-     for packed_field in byte.packed_fields %}
     encoder.{{packed_field.field.kind|encode_snippet}}val.{{packed_field.field.name}});
 {%-     endfor %}
 {%-   endif %}
 {%-   if byte.is_padding %}
     encoder.skip(1);
 {%-   endif %}
 {%- endfor %}
   };