mojo-js: Add support for nested enums in interfaces

We weren't generated enum definitions for nested enums in interfaces.

Change-Id: Ib303509dd991f0555ceb4e092ce41a4da91d3640
Reviewed-on: https://chromium-review.googlesource.com/c/1350440Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610957}

mojo/public/tools/bindings/generators/js_templates/lite/interface_definition.tmpl

@@ -142,6 +142,12 @@ class {{interface.name}} {
   }
 }
 
+{#--- Enums #}
+{% from "lite/enum_definition.tmpl" import enum_def %}
+{%- for enum in interface.enums %}
+{{ enum_def("%s.%s"|format(interface.name, enum.name), enum) }}
+{%-  endfor %}
+
 /** @const {!string} */
 {{interface.name}}.$interfaceName = '{{mojom_namespace}}.{{interface.name}}';