Implements gr-reporting as common reporting interface.
Adds UI latency reporting methods:
- time and timeEnd to measure individual events.
- appStarted and pageLoaded to report startup metrics.
- reporter property at the moment reports metrics via DOM CustomEvents.
UI Switcher extension listens to those and reports them to Google
Analytics.
Also, see https://gerrit-review.googlesource.com/83512
Also, go/gerrit-reporting
Change-Id: I36a166f07be281761262fefa50cc539cc6ea2d19