Kill scattered extern _PDF_CharType declarations.

While we're at it, avoid an indirection through a pointer, and
use a name that isn't reserved for the compiler (leading _ CAP).
This is a small portion of the associated bug:

BUG=https://code.google.com/p/pdfium/issues/detail?id=112
R=brucedawson@chromium.org

Review URL: https://codereview.chromium.org/880663003