WorkerThreadableWebSocketChannel::send doesn't have to wait for the main thread.

WorkerThreadableWebSocketChannel::send calls NewWebSocketChannelImpl::send
asynchronously. Currently the former waits for the completion of the latter
operation, but it is not necessary because NewWebSocketChannelImpl::send
does not return a meaningful failure.
This CL also replaces never taken |if (xxx) return| statements with ASSERT(!xxx).

BUG=397403

Review URL: https://codereview.chromium.org/427783003

git-svn-id: svn://svn.chromium.org/blink/trunk@179220 bbb929c8-8fbe-4397-9dbb-9b2b20218538