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As a first step of supporting back-forward cached navigations for
PageLoadMetricsObservers, do not track them at all.

Design doc:
https://docs.google.com/document/d/1VxjalqgiDvoAxQBGetBQi2m179UmiUWqftqq1XBRSWs/edit#heading=h.lacw135tkjlp
R=bmcquade@chromium.org,alexmos@chromium.org
BUG=1014174

Change-Id: Ic96914a5c72d7a2619948fb1f39e388f4311f004
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860416Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705895}