Merge to XFA: Move extern fpdf_font_charset.cpp function prototypes. (cherry picked from commit c5fa7f2a36ebd4c92c935e1332a2f7db4f3594d9) Orignal Review URL: https://codereview.chromium.org/1241173004 . TBR=thestig@chromium.org Review URL: https://codereview.chromium.org/1243243003 .