Intent to Ship: allow pages with *unload event listeners to enter bfcache on Android

[smaug]

Hi all,

in order to let more pages to utilize bfcache, pages with *unload event listeners will not be blocked from being bfcached anymore. This time the change is only for Android and only for cross-domain navigations in order to have somewhat similar behavior what some other mobile browsers have.

At the same time there is a new restriction: pages with an opener will not enter bfcache. Fission will have this same behavior.

https://bugzilla.mozilla.org/show_bug.cgi?id=1686783

The bug has links to relevant (mochi)test changes.

WPT testing of bfcache is still somewhat limited since not all the browsers have supported bfcache, but the situation is improving. See for example

https://github.com/web-platform-tests/wpt/issues/16359

-Olli

[James Graham]

Note that there's a Chromium CL open that will upstream some of their
tests to wpt [1]. I don't know to what extent we agree with those tests,
but I do think that by sharing tests early in the process we're more
likely to end up in a place where any implementation differences are
well understood and we avoid creating unexpected web-compat issues.

[1] https://github.com/web-platform-tests/wpt/pull/28324