This fixes tests that block me from landing bug 1847584. In particular,
consider that we set a tab to `display: none` (e.g. by entering
customize mode). Only then, we try to switch tabs.
There can be the case that painting is fast enough in clearing our
layers, and by the time we set renderLayers = false, we already do not
have layers (from display: none clearing them).
This makes transitioning to UNLOADING/LOADING potentially transition
directly to LOADED/UNLOADED, but that seems fine. That's already the
case for non-remote browsers.
This makes the remote and non-remote code-paths more similar, which is
nice.
Differential Revision: https://phabricator.services.mozilla.com/D187387
cbe72a6d2e