Mark a couple of fast/repaint/ tests as flaky

The following tests are flaky after r167542

fast/repaint/selection-after-remove.html
fast/repaint/selection-gap-overflow-scroll.html
fast/repaint/delete-into-nested-block.html

TBR=jamesr

Review URL: https://codereview.chromium.org/175043003

git-svn-id: svn://svn.chromium.org/blink/trunk@167590 bbb929c8-8fbe-4397-9dbb-9b2b20218538