Add additional PageLoad UKM metrics using new UKM client API.

This change adds core PageLoad metrics to UKM. The new metrics are:
* first meaningful paint (experimental)
* foreground duration
* parse start
* dom content loaded
* load event

BUG=697454

Review-Url: https://codereview.chromium.org/2727143004
Cr-Commit-Position: refs/heads/master@{#454308}
3 files changed