Remove a deprecated WebSocketHandleClient method.

WebSocketHandleClient::didClose(WebSocketHandle*, unsigned short, const WebString&)
is deprecated and was waiting for chromium side code to stop using it.
Now the chromium side change is done, so delete it.

cf:
 - https://codereview.chromium.org/36033003
 - https://codereview.chromium.org/34753008

BUG=310405
R=abarth

Review URL: https://codereview.chromium.org/61433003

git-svn-id: svn://svn.chromium.org/blink/trunk@161470 bbb929c8-8fbe-4397-9dbb-9b2b20218538