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Requests that the sampling profiler apply metadata to samples between
navigation start and first contentful paint, at the time when first
contentful paint is sent to the browser process. This metadata will
support analysis of execution during page load.

This change introduces monotonic time interfaces for the required metrics
because the existing interfaces are limited to 1ms resolution. If used
they would result in up to 2% of samples being misidentified as occurring
during loading when they weren't, or vice versa.

This is a reland of http://crrev.com/742717 and adds logic to avoid
applying metadata on time ranges that don't make sense but are
occasionally seen in practice.

TBR=npm@chromium.org,schenney@chromium.org

Bug: 1034756, 1054357
Change-Id: Ie7e7a26b4e2c86219eb30dd678e5de296d2f59be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067287Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744852}