We need the key to be a String even though we claimed we could support any
Object. Also, clean up some style nits including shortening the |root| getters
on OneChildRenderNodeWrappers.

Fixes https://github.com/domokit/sky_sdk/issues/26.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1173293005.