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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.

Bug: 1034756
Change-Id: Ibb39877ecfc52a1e5327204c39c3b0ce0f1b8ccd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042324Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742717}