autohiding toolbars: Wait for [sessionrestored] before setting urlbar transition
--urlbar-container-height, which is set for #urlbar-container, is computed
dynamically early on startup. If we set transition to it, then it will transition
from the initial height to the new value and because of that the computed value
is determined using the initial height instead of the "correct" value set by
toolbar density options. The issue most noticeably occurs when initial window
has startup page as blank, and when compact density is selected, but it does
but really it can also happen without them, the effect just might be so subtle
that it's not perceivable.