Bug 1782597 Part 2 - Use null to indicate page value equal to the auto value for...

Bug 1782597 Part 2 - Use null to indicate page value equal to the auto value for CSS named pages r=dholbert

This applies both to the individual mStartPageValue and mEndPageValue fields
of the nsIFrame::PageValues struct, and for the nsIFrame::PageValuesProperty
being null to indicate both mStartPageValue and mEndPageValue are auto.

Fetching this is handled by nsIFrame::GetStartPageValue and
nsIFrame::GetEndPageValue, which also ensure the use of first-in-flow frames.

Differential Revision: https://phabricator.services.mozilla.com/D157873