Sync web-platform-tests PR 25466 into mozilla-central (this bug is closed when the sync is complete).
Fix double zooming in StyleBuilderConverter::ConvertBorderWidth()
StyleBuilderConverter::ConvertBorderWidth() is computing the zoom twice,
the value we set in |result| variable is already zoomed. But then we
have some extra lines that zoom it again and check if the final value is
between 0px and 1px, to return 1px.
This is wrong, as |result| was already zoomed, we can avoid that second
operation and use that value to do the check directly.
This was causing issues in the past but in very corner cases (so not
very easy to reproduce). However due to the change on how subpixel
border widths are managed (now they're floored, see r770064) this issue
is causing that some borders are invisible.
Added one WPT test that would fail with scaling factor 1.5.
Also added a internal test using "zoom" property.