Implement WebSocket blocking

[tomasko126]

Enable support for blocking WebSocket connections, as currently AB and ABP support it.

[tomasko126]

A Chromium bug has been created for WebSocket blocking implementation. Related patch, which brings WebSocket blocking as part of the webRequest API should land soon into Chromium trunk.

[tomasko126]

@kpeckett WebSocket blocking is now natively supported in Chrome v58, according to the following bug report.