AdsPageLoadMetricsObserver continues to monitor after reporting.

The current behavior of AdsPageLoadMetricsObserver (APLMO) is to stop
observing when chrome is backgrounded and report all histograms because
it is unknown if proper reporting will occur.  While this behavior is
desired, it also means that the observer stops monitoring the page,
preventing it from properly triggering when a Heavy Ads intervention
occurs.  This change finalizes histograms when chrome is backgrounded,
but has APLMO continue observing after that time.

This also adds testing around FlushMetricsOnAppEntersBackground
functionality, as no testing of that existed previously for the APLMO.

Bug: 1103699
Change-Id: Ib4f5eeb0b90a30e3189e0490cbb13df5b1e49e43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2305675
Reviewed-by: John Delaney <johnidel@chromium.org>
Commit-Queue: Eric Robinson <ericrobinson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801560}