Notes:
* In CSSAlignUtils.h, I'm just adding a forward-decl for a type used there.
* RubyUtils.h needs to #include nsIFrame.h -- a forward-declare won't
suffice -- because it makes a nsIFrame method-call ("aFrame->GetType()").
* ScrollSnap.h uses the "Maybe" type and a unit defined in
nsIScrollableFrame.h; hence it needs both of those headers.
MozReview-Commit-ID: KuhJVh9RTjZ
--HG--
extra : rebase_source : 062c1abfe9a1efeb7e241f38302c140075d2ae88