style: Make zoom: 0 mean the same as zoom: 1.
style: Make zoom: 0 mean the same as zoom: 1.

This matches the WebKit implementation, and is clearly a violation of the rules
we generally use for ranges in CSS.

But it seems to be depended-on legacy behavior, see the linked WebKit bug, this
bug, and bug 1593317.

Differential Revision: https://phabricator.services.mozilla.com/D51539

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