If we set m_waiting_for_messages to true for the intial message request,
we might never receive a response, and block the console from requesting
any more messages indefinitely. Instead, issue a non-blocking initial
request, which is how the Qt and Serenity chromes behaved before the
ConsoleClient abstraction.
This also changes ConsoleClient::clear() to use run_javascript() rather
than js_console_input(), as that is also how the chromes used to behave.