FrameManager.OnClientDisconnectAsync used to race a 200ms timer against
the FrameSwappedByActivation event, so a slow prerender activation could
be treated as a disconnect and tear down frames that were actually about
to be swapped in. It also didn't track which CDPSession disconnected, so
a second disconnect event on an already-swapped session could remove
frames belonging to the new session.

Now the wait has no timer: it waits until either the swap event fires or
the page closes, and it bails out early if the disconnected client isn't
the current one (meaning a swap already happened). Also treats a closed
page the same as a disconnected browser, since activation can't happen
either way.

Updated the prerender test assets/assertions to actually verify whether
a page was served from the prerender cache (via a small detection script
in target.html) instead of just checking body text, which stayed the
same either way and couldn't catch this race.

Upstream: puppeteer/puppeteer#15198