We maintain a simple LRU cache of tab layers by setting their
docShellIsActive = false with preserveLayers(true). Once they
are pushed out of the cache by more recently used tabs, their
layers are discarded.
Luckily most of the complexity of this could be contained in
the AsyncTabSwitcher - the one change that had to sit outside of
that was moving the aTab.closing = true earlier in the removeTab
call, so that we could use that information to eagerly evict tabs
from the cache. This was to address a leak in a few tests on try.
MozReview-Commit-ID: 2E3uU8LEYkD
db8bf8252e