Bug 1598145 Part 1: For user-scalable=no, force min and max zoom to match initial zoom, add WPT. r=hiro

This provides web compatability with Chrome's interpretation of the spec at
https://drafts.csswg.org/css-device-adapt/#user-zoom-desc. This changes makes
us treat "the user cannot interactively change the zoom factor" as inclusive
of ANY method of the user agent changing zoom levels, regardless of whether or
not the user initiated an action.

So, in addition to preventing things like double-tap-to-zoom and pinch-zoom,
this change prevents zoom changes to fit all content into the viewport, or to
maintain visible content proportionality if the viewport is resized dynamically.

This also adds an assert to cases where Document::GetViewportInfo returns a
viewport with a non-positive scale factor that it also sets the auto scale
flag.

This also adds a tentative web-platform test.

Differential Revision: https://phabricator.services.mozilla.com/D72004