Clear mouseEventQueue between test runs

Clear the mouseEventQueue in EventSender between test runs.

BUG=304996
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/26467008

git-svn-id: svn://svn.chromium.org/blink/trunk@159397 bbb929c8-8fbe-4397-9dbb-9b2b20218538