Ad metrics: Fix AdPaintTiming.NavigationToFirstContentfulPaint.

Currently the FCP ads metric only records the FCP of the main frame and
ignores the FCP of any child frames. This CL updates the metric to
store the earliest FCP in the ad frame tree, taking into account the
child frames as well as the root ad frame.

Tests:

chrome/test:unit_tests
--gtest_filter=*FirstContentfulPaint*

Bug: 1103782
Change-Id: Id1cb1e9a98ba03656bb3da1ef6592b779bd5fb10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307442
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: John Delaney <johnidel@chromium.org>
Commit-Queue: Cammie Smith Barnes <cammie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791523}