DevTools: remove most of runAfterPendingDispatches calls from styles tests

The patch removes most of the runAfterPendingDispatches calls from styles
tests. This also fixes flakiness of styles-update-links.html.

R=vsevik,apavlov

Review URL: https://codereview.chromium.org/626843002

git-svn-id: svn://svn.chromium.org/blink/trunk@183194 bbb929c8-8fbe-4397-9dbb-9b2b20218538