Actor: Improve logging in PageStabilityMonitor

There are two issues with journal logs in the PageStabilityMonitor:

First, the renderer-side journal isn't currently synced back to the
browser process if the renderer enters the BFCache (or similar page
freezing scenarios). Ensure we flush the buffer in this case so logs
aren't dropped when this happens.

Second, the actor journal is currently bound from PageTool which means
that until a PageTool is invoked the journal (e.g. from
PageStabilityMonitor) in the renderer isn't captured. Fix this by also
binding a journal in ObservationDelayController when
PageStabilityMonitor is used.

To make journals more readable, replace ad-hoc logs with an entry on
each state-change (like ObservationDelayController). These entries are
made async to make state changes clearer in a trace UI and we use state
names rather than the underlying int type.

Bug: b:434268231
Change-Id: I98ff9c1470e4a447de2109c42e9811703c564b4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6997689
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523969}