DevTools: fix flakiness of some inspector/elements/styles tests

The flakiness was implied by interference of throttled process applyStyleText
and committing of a new style rule. Helper method relied on
InspectorTest.runAfterPendingDispatches method, which in this case turned
out to be invalid.

BUG=416846, 418483
R=pfeldman, apavlov

Review URL: https://codereview.chromium.org/617473002

git-svn-id: svn://svn.chromium.org/blink/trunk@183047 bbb929c8-8fbe-4397-9dbb-9b2b20218538