chromium: Enable pulseaudio by default.

If no config.pulseaudio is explicitely set to false, build with pulse
support, because even if there is no pulse server available, chromium
will fall back to using ALSA.

And we definitely want to avoid that users have to build chromium for
themselves just for the sake of having pulse support. Thanks to @devhell
for actually helping me discovering this (I for myself do always rebuild
Chromium, so I won't notice those kind of things).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>