www/firefox: re-enable OMTC for BUNDLED_CAIRO=off (backing out r393805)

OMTC is also required to support Electrolysis (called Multiprocess Windows in
about:support) introduced in Firefox 48 via e10srollout@mozilla.org system addon.

PR: 202174, 211792
Tested by: scf
MFH: 2016Q3