[Extensions] Force the GLib main-loop on Linux platform

Many Linux external extensions depends on glib mainloop integration,
mainly for DBus. GLib is the default main loop on Linux Desktop, but
when building if Ozone, you get a Libevent mainloop. This patch
brings GLib back to extension process (only) when building Ozone.

Should not affect anything when building with X11 support.

BUG=XWALK-1331