There's an Intent to Ship for this on blink-dev:
Please shout if shipping this now is not a good idea.
This has now landed in Chrome 64: https://bugs.chromium.org/p/chromium/issues/detail?id=612962
It'd be super cool to get this in Webkit/Safari :-).
This is extreme helpful for developers (https://developers.google.com/web/updates/2016/10/resizeobserver). Unfortunately polyfills are not enough here. Please provide support for this functionality.