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What:
Create a PageTimingSender for frames that abort their initial navigation
with a doc.write to render content. This is done by observing
document element without a committed load.

Why:
This behavior has been seen numerous times in the wild causing, where
PLM byte metrics are not catching the majority of network usage by the
page. In a number of instances, there are video players loaded inside of
these frames causing most network bytes to be unattributed to the
PageLoad.

This also affects the HeavyAdIntervention, which uses PLM measurements
to trigger unloading.

How:
For these types of loads, we will not send paint timing, render data,
or metadata updates to the browser process. There is currently a lack
of understanding regarding paint timing in frames like this. There is
also no browser process machinery to merge timing updates in frames that
do not have loads (as we cannot get a navigation start offset).

It is unclear the effect that this change will have overall on metrics.
Anecdotally, it is not expected to not produce a significant change due
to being a fairly nuanced edge case.

Bug: 914893
Change-Id: Ic696e693e11f2d7cdcd8c97e9418a55d7bf4b2a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1822218Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782709}