[CSS Grid Layout] 2 span positions are not resolved correctly

The resolution code in resolveGridPositionsFromStyle had a bug where
we would check for 2 'auto' positions to determine if we needed to
run the auto-placement algorithm. However the specification considers
other positions as being treat as 'auto' depending on the context.
Due to this glitch, we would try to explicitly resolve a 'span'
position yielding to ASSERTs and the wrong result.

While touching some grid parsing tests, added a real grid container
so they now also test the resolution code path. Also removed some
unneeded internal overrides.

TEST=grid-item-bad-resolution-double-span.html

Review URL: https://chromiumcodereview.appspot.com/21564004

git-svn-id: svn://svn.chromium.org/blink/trunk@155397 bbb929c8-8fbe-4397-9dbb-9b2b20218538