Extend flakiness of overflow/overflow-positioning.html.

Sadly, it's flaky occasionally everywhere.

BUG=245630
TBR=enne

Review URL: https://codereview.chromium.org/47923004

git-svn-id: svn://svn.chromium.org/blink/trunk@160731 bbb929c8-8fbe-4397-9dbb-9b2b20218538