Audit: Content scrolls at 60fps

What we want:

* Ability to scroll/fling the page as similar to a mobile scroll/fling as possible
* Gather data from trace and compute a [Frame Throughput](https://docs.google.com/document/d/1Bot91txCWZUstt32_BBLo-_HUsvOVn_L5yaI5xre2M0/edit#heading=h.o2z4mecc57o8) ([crbug](https://bugs.chromium.org/p/chromium/issues/detail?id=625700)) or other similar smoothness metric

A very nice PR from @surma was put up in https://github.com/GoogleChrome/lighthouse/pull/469
Since this PR, we've done a lot of work internally to handle gathering a second trace and passing it off correctly, so we're in better shape now.