These don't get styles early enough to make a shadow decision before Show().
This also matches the previous behavior (nothing would set
nsWidgetInitData::mDropShadow for these windows), so this is the less risky
fix.

It seems somewhat sketchy to use a popup window for these to begin with (Linux
for example maps them to a toplevel Window, at least on Wayland...), but let's
not change too much right now.

The hbrBackground change is a no-op because we can't have a brush before
Create() is called, we update it in SetBackgroundColor().

Differential Revision: https://phabricator.services.mozilla.com/D153095