webkitgtk: Allow toggling WebDriver support

Version 2.18.x includes support for remotely controlling WebKitGTK+
based browsers using the standard WebDriver API. Typically this is used
by Web developers, and in most cases it will be desirable to disable it
from builds.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>