Tracking bug for this work.

Current blocker - security concerns around exposing third party resource sizes. It looks likely that we will resolve this concern without changing the API surface. If that is the case, then Chrome is OK to move forward since we already expose 3rd party resource sizes with the API in the document. 

jsbell is talking with MSFT at SW f2f and I will talk to Mozilla this week to see if they have concerns around shipping the api (and essentially freezing it, since developers will start using it)