Moving clock timeout flakiness guard in test to 20 ms

Up to 10 ms backwards clock jumps has been seen on XP, 2 ms on Win7.

BUG=282620

Review URL: https://chromiumcodereview.appspot.com/23648005

git-svn-id: svn://svn.chromium.org/blink/trunk@157448 bbb929c8-8fbe-4397-9dbb-9b2b20218538