[CSS Grid Layout] Ignore positioned block item added after inline

Right now if you add a positioned block item after an inline item, the
positioned block item is inserted inside the anonymous block wrapping
the inline item.

This was causing an assert in RenderGrid::addChildToIndexesMap() while
navigating the sibling boxes, as its previous sibling is an inline.

As the positioned block is now not direct child of the grid, we can
ignore it, as it's not considered a grid item. The grid item would
be the anonymous block. So the proper check is added in
RenderGrid::addChild to do it.

BUG=401463
TEST=fast/css-grid-layout/grid-add-positioned-block-item-after-inline-item.html

Review URL: https://codereview.chromium.org/450783002

git-svn-id: svn://svn.chromium.org/blink/trunk@181633 bbb929c8-8fbe-4397-9dbb-9b2b20218538