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
(The only reason this isn't triggering a compile error is due to .cpp files'
"using namespace mozilla" getting haphazardly shared via unified builds.)
MozReview-Commit-ID: 1zhJueMWOav
