…-size.

Per the css-contain specification, size contained elements must be sized as if
they were empty. Up until now, we've been handling that by just using "0" as
the intrinsic size of some components, but that doesn't actually match the size
of a "true" empty select, which has some nonzero width from:
 (a) the default inline-axis padding on the display frame (added in a rule for
     the ::-moz-display-comboboxcontrol-frame pseudo, in forms.css).

 (b) the width (inline-size) of the display frame's "placeholder" space
     character, which has a small intrinsic width (but which really only exists
     for *block-axis* sizing and alignment, when no option is selected from
     the dropdown).

This patch addresses issue (a) by explicitly adding the display frame's
inline-axis padding to size-contained elements, and it addresses issue (b) by
changing to use a zero-width space character in empty select elements.

So: as of this patch, size-contained select elements are getting a little wider
(to address (a)), and empty select elements are also getting a little skinnier
(to address (b)), and they'll end up being the same width.

(I chose U+FEFF "zero-width non-breaking-space" since we were previously using
a non-breaking space character. I'm not sure if the non-breaking aspect matters,
but I figured I'd preserve that to be on the safe side.)

Differential Revision: https://phabricator.services.mozilla.com/D48791

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1562057
gecko-commit: 25d95be82e953bfe3758acb96c3fb80b9b602f5a
gecko-integration-branch: autoland
gecko-reviewers: TYLin