Add and report LayoutStability.JankScore UKM.

We prolong the lifetime of UkmPageLoadMetricsObserver to permit jank
score reporting at session end for tabs that start or become hidden,
but avoid extra reporting of other metrics by checking was_hidden_.

The OnFinalLayoutStabilityUpdate hook is removed in favor of passing
PageRenderData to existing hooks through PageLoadExtraInfo.  This lets
the observer decide when to report.

UKM collection review doc: go/lsukmrev

Bug: 581518
Change-Id: I2983dff155872f3080474e09df784d20b3cb08d3
Reviewed-on: https://chromium-review.googlesource.com/c/1299917
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606877}
7 files changed