[CSSGridLayout] Cleanup resolveContentBasedTrackSizingFunctionsForItems
[CSSGridLayout] Cleanup resolveContentBasedTrackSizingFunctionsForItems

We can use GridSpan iterator to simplify the code that transverses each
item's resolved positions. Apart from that we can easily remove one loop if
we compute the sum of the sizes of the tracks an item spans through at the
time of filtering them.

Review URL: https://codereview.chromium.org/865443006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@189628 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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