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Turns out it's not very accurate at all and often gets misinterpeted as
representing user behaviour.  Deprecate it and point to
PageLoad.Timing2.NavigationToCommit which is more accurate for user
behaviour.

See also: https://docs.google.com/document/d/1TUvbPhbcuKGPrwx1iqUIisiV9YVWrz_SDTfdEz0_SDM/edit?ts=5ebc27cc#

Change-Id: Ic748fa3f3722237b5628353f680a13d704ff5840
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2198684Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780076}