stylo: Don't serialize default position on -moz- prefixed linear gradient
Normally, we should not serialize if the line direction points downwards. Unprefixed and
-webkit- prefixed syntaxes use only Keyword enum for line direction and we check them already.
But -moz- prefixed linear gradients can store position too. Therefore it's stored inside
`LineDirection::MozPosition` and we should check that for consistency between other gradients.
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- [X] These changes fix [Bug 1396102](https://bugzilla.mozilla.org/show_bug.cgi?id=1396102)
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