Autoplay time metric

We're going to run an experiment to make the "canplaythrough" event fire according
to spec instead of at the same time as "canplay" this will sometimes delay autoplay,
and we want to know how much, so let's add a metric for that.

BUG=73609

Review-Url: https://codereview.chromium.org/2856783002
Cr-Commit-Position: refs/heads/master@{#472331}
4 files changed