[Mojo] Make javascript enums extensible.

mojo/public/tools/bindings/generators/js_templates/enum_definition.tmpl

 {%- macro enum_def(enum_name, enum) -%}
   {{enum_name}} = {};
-{%- set prev_enum = 0 %}
-{%- for field in enum.fields %}
-{%-   if field.value %}
+{%-   set prev_enum = 0 %}
+{%-   for field in enum.fields %}
+{%-     if field.value %}
   {{enum_name}}.{{field.name}} = {{field.value|expression_to_text}};
-{%-   elif loop.first %}
+{%-     elif loop.first %}
   {{enum_name}}.{{field.name}} = 0;
-{%-   else %}
+{%-     else %}
   {{enum_name}}.{{field.name}} = {{enum_name}}.{{enum.fields[loop.index0 - 1].name}} + 1;
+{%-     endif %}
+{%-   endfor %}
+
+  {{enum_name}}.isKnownEnumValue = function(value) {
+{%-   if enum.fields %}
+    switch (value) {
+{%-       for enum_field in enum.fields|groupby('numeric_value') %}
+    case {{enum_field[0]}}:
+{%-     endfor %}
+      return true;
+    }
 {%-   endif %}
-{%- endfor %}
+    return false;
+  };
+
+  {{enum_name}}.validate = function(enumValue) {
+    var isExtensible = {% if enum.extensible %}true{% else %}false{% endif %};
+    if (isExtensible || this.isKnownEnumValue(enumValue))
+      return validator.validationError.NONE;
+
+    return validator.validationError.UNKNOWN_ENUM_VALUE;
+  };
 {%- endmacro %}