[CSS Grid Layout] Add GridSpan::iterator

As suggested when GridResolvedPosition was introduced, this patch
adds a GridSpan::iterator which allows to simplify the loops over
GridResolvedPositions.

BUG=258258

Review URL: https://codereview.chromium.org/235663004

git-svn-id: svn://svn.chromium.org/blink/trunk@172404 bbb929c8-8fbe-4397-9dbb-9b2b20218538