Fix selection bg in network panel.

Bonus fix: 10x paint time boost for other DataGrids.

BUG=338254

Review URL: https://codereview.chromium.org/148173002

git-svn-id: svn://svn.chromium.org/blink/trunk@165859 bbb929c8-8fbe-4397-9dbb-9b2b20218538