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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}