Heap-use-after-free in RenderGrid::computePreferredTrackWidth

This issue is that the out-of-flow handling logic could remove
anonymous block grid items without notifying the grid container.
The grid container would thus hold onto a stale pointer.

BUG=280451

Review URL: https://chromiumcodereview.appspot.com/23890003

git-svn-id: svn://svn.chromium.org/blink/trunk@157172 bbb929c8-8fbe-4397-9dbb-9b2b20218538