Add observer and handler for page content cache

Implement observers for keeping the cache updated. No new triggering for
extraction. This only works if the service extracts content on every
page load. Future changes will add more extraction triggers, like
backgrounding.

Observe for page load, tab backgrounding from webcontents observer.
Observe for tab closures from android tab model observer, since
webcontents are destroyed more often on android.
Observe for content extraction and cache for background tabs.

Fix observer to notify request on visibility changes. Revert the delay
handling for hidden tabs - causes race condition with unrelated test.

Tests: unitests are not feasible with tab observation. Browser test
will be added in follow up.

Bug: 440643544
Change-Id: Ice179756da56020ae0eabe6c02d10fa68701ba5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6957164
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Commit-Queue: Siddhartha S <ssid@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525723}