Add fallback for non-embedded "Century Gothic" CIDFontType2 font (issue 4722 and bug 879561)

According to practical experiments, falling back to "Helvetica" when we encounter a non-embedded "[Century Gothic](http://en.wikipedia.org/wiki/Century_Gothic)" `CIDFontType2` font seems to work well.
(Also, the section on Wikipedia about "Printer ink usage" *might* provide some anecdotal evidence that Century Gothic is a fairly standard sans-serif font.)

Obviously this patch doesn't make "Century Gothic" fonts render perfectly, as is often the case with non-embedded fonts, but all the text is now legible in the referenced issues.

Fixes 4722.
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=879561.