style: Make the simplified children iterator check account for Shadow DOM on its own.
style: Make the simplified children iterator check account for Shadow DOM on its own.

We are relying in the XBL binding for the shadow host, we need to check
explicitly.

Also, we were checking in_shadow_tree, which is really broad. Just
is_html_slot_element should do, since the rest of the nodes should have only
light tree children.

Bug: 1425759
Reviewed-by: xidorn
MozReview-Commit-ID: KAFRVxaLsK

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