Tab unloading should not unload "fresh" tabs i.e. tabs that were accessed
very recently, even though one of them was the least recently used tab.
To achieve it, this patch introduces the minimum inactive duration and we
prevent tabs that were accessed in the last period of that duration from
being unloaded.
This patch sets the default value to 10 minutes based on the historgram
`TAB_UNLOAD_TO_RELOAD` where the median was 60sec and the 75th percentile
was 1040sec. This value can be overriden by the parameter of the TabUnloader's
APIs so that about:unloads and the tests can unload those fresh tabs.
Differential Revision: https://phabricator.services.mozilla.com/D125824
f1e6437e96