DevTools: Fix test inspector-protocol/stylesheet-tracking-restart.html flakiness

The flakiness is caused by unpredictable order of stylesheets binding in InspectorCSSAgent.
This patch replaces HashSet with Vector to make sure the order is maintained.

R=lushnikov

Review URL: https://codereview.chromium.org/339133002

git-svn-id: svn://svn.chromium.org/blink/trunk@176301 bbb929c8-8fbe-4397-9dbb-9b2b20218538