Make sure run-perf-tests script does not fail due to console messages

Make sure run-perf-tests script does not fail on console messages (e.g.
deprecation warnings). This was reproduceable with Dromaeo/dom-traverse.html
as it is using the deprecated Event.returnValue attribute.

R=marja, dpranke
BUG=304800

Review URL: https://codereview.chromium.org/26473005

git-svn-id: svn://svn.chromium.org/blink/trunk@159176 bbb929c8-8fbe-4397-9dbb-9b2b20218538