Fix coding style violation in UserGestureToken's ctor

BUG=none
TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/274563010

git-svn-id: svn://svn.chromium.org/blink/trunk@173638 bbb929c8-8fbe-4397-9dbb-9b2b20218538