When comparing a Maybe<WrScrollId> to another WrScrollId we need to properly
handle the case where Nothing() signifies the root scroll frame. This is
equivalent to calling scrollId.valueOr(FrameMetrics::NULL_SCROLL_ID), as was
done before WrScrolLId replaced ViewId in the WebRender ScrollingLayersHelper.
We also have DisplayListBuilder::TopmostScrollId always return a value instead
of a Maybe, since an empty clip stack means that the current scroll id is that
of the root scroll frame.
Previously Nothing() was not equivalent to WrScrollId { 0 }, which caused the
ScrollingLayersHelper to fill the mClipAndScroll value and push another
set of clip and scroll nodes onto the WebRender display list builder.
MozReview-Commit-ID: CeatZlRXtuI
Martin Roinson