Stop sending 2 Close frames from client-close_wsh

client-close_wsh.py generates one Close frame directly, and then
pywebsocket sends another one. Tell pywebsocket that the Close frame has
already been sent.

BUG=345683

Review URL: https://codereview.chromium.org/177223002

git-svn-id: svn://svn.chromium.org/blink/trunk@167899 bbb929c8-8fbe-4397-9dbb-9b2b20218538