[WebSocket] Add Sec-WebSocket-Extensions header to handshake response.

Add the header to the handshake response in order to show it in the inspector.

BUG=229290
R=tyoshino

Review URL: https://codereview.chromium.org/31453003

git-svn-id: svn://svn.chromium.org/blink/trunk@160069 bbb929c8-8fbe-4397-9dbb-9b2b20218538