Remove fail argument from WebSocket AddChannelResponse message

This reduces the number of messages. We can still see whether a received
failure notification is for handshake or after handshake by checking their
own state.

BUG=none

Review URL: https://codereview.chromium.org/950903004

Cr-Commit-Position: refs/heads/master@{#319412}
13 files changed