Close accept_event_ only after stopping accept_watcher_.

See also https://codereview.chromium.org/60853003/

R=kkania@chromium.org,yzshen@chromium.org
BUG=chromedriver:573
TEST=none

Review URL: https://codereview.chromium.org/79693003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236655 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236655