[DO NOT MERGE] - Attempt 1 at fixing the generator

The generator works via using postMessage to the testharness_timing and getting responses. 

I think this method turned out to be overly complicated but has a few nice properties;
 * tests already in the file are run, trivial to add a setting to generate checks at those locations too.
 * postMessage support leads way for other interesting control mechanisms and stuff.

It depends on the setting framework in https://github.com/web-animations/web-animations-js/pull/305

Not sure if it's overall positive or not..