Dealing with percent character and Google translate (bug 1007870)

A follow-up of the previous quick fix: https://github.com/mozilla/olympia/commit/49867fbd8813ee73bff83c7651513fa85b486f5a

Note that it doesn't work with Firefox but it's OK with Chrome, see details and STR on https://bugzilla.mozilla.org/show_bug.cgi?id=1007870