Have FontFallbackList::primaryFontData always return font data

Have FontFallbackList::primaryFontData always return font data

The current implementation of FontFallbackList::primaryFontData can
return NULL in certain cases however the majority of callers assumes
that it never does.

Change the implementation to return the last resort fallback if no
other suitable font can be found.

This is a speculative fix for the top linux crash report.

BUG=314128
R=schenney@chromium.org

[dglazkov, 2013-11-07 18:15:05]

lgtm. I think not ever returning 0 is reasonable (maybe it should return a
reference or something?). I have no clue whether
getNonRetainedLastResortFallbackFont is the right thing to return though.
Doesn't hurt to try, right? :)