Add LCP and OnLoad to AbandonedPageLoadMetricsObserver

This CL adds LCP and OnLoad events to the abandoned PLMO.

Adding OnLoad event is straightforward, but LCP is a bit tricky because
the metrics is collected in either OnComplete or
FlushMetricsOnAppEnterBackground in PLMO, which is not invoked during
the regular page load. At the moment this CL only has a very simple
test.

Change-Id: I49adf28ebe26b6767a94e47499157d4bf414ad72
Bug: 347706997
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5725416
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1331473}