webperf: log per-user-agent Navigation Timing data
- Parse user-agent strings using a custom set of regular expressions, because the set of browsers which implement Navigation Timing is a small subset of the set of browsers that user-agent parsing libraries handle. The regular expressions in this patch were tested against a log of user-agent strings from NavigationTiming events.
- Drop '_with_zero' metrics, added for T104902.
Data will be available under the metric hierarchy