Adding the start of a statistics widget in the nav bar.
Right now it shows stats about FPS and GCs. This also adds shared indices
to zones so that ones like the #gc don't get regenerated multiple times.

Partial work on #279. There's no histogram yet (pending #263), but the
stats are real.
Merge pull request #285 from google/statsbox

Adding the start of a statistics widget in the nav bar.