Add UseCounters for beforeload event and getMatchedCSSRules.

Review URL: https://codereview.chromium.org/25364004

git-svn-id: svn://svn.chromium.org/blink/trunk@158849 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Clear debug overlay message before issuing step/resume.

BUG=302534
R=pfeldman,yurys

Review URL: https://codereview.chromium.org/25849003

git-svn-id: svn://svn.chromium.org/blink/trunk@158850 bbb929c8-8fbe-4397-9dbb-9b2b20218538