Refactor grid size estimation to use resolveGridPositionsFromStyle
Refactor grid size estimation to use resolveGridPositionsFromStyle

resolveGridPositionsFromStyle required to have the grid filled so that
we resolve negative indexes against the grid's final edge. The
specification (and the code) changed to use the 'explicit grid' which
is independent of the grid items and depends only on the style.

This means that we can now safely reuse resolveGridPositionsFromStyle
during grid size estimation, which will make it easier to implement
'span' handling (along with removing some code complexity).

BUG=229713
TEST=All grid layout tests passing

Review URL: https://chromiumcodereview.appspot.com/14401010

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@148878 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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