Don't try to paint selection gaps between flex items.

The selection gap code is designed for children flowing vertically.  When it tries to process a horizontal flex item, the gap can paint over an adjacent item.

BUG=381987

Review URL: https://codereview.chromium.org/350713004

git-svn-id: svn://svn.chromium.org/blink/trunk@176874 bbb929c8-8fbe-4397-9dbb-9b2b20218538