AFAICS there's no need to change anything in the spec:
```attribute EventHandler        onchargingchange;```
is the corresponding IDL line.

This allows both[1], the legacy assignment and the "addEventListener" syntax - with the latter being recommend, the former being kind of deprecated. (There's IDL support for this when implemented inside Blink - I am not sure how an EventHandler IDL type is handled in Crosswalk extensions.)

[1] Cmp. http://www.w3.org/html/wg/drafts/html/master/webappapis.html#event-handler-attributes

Does this help?