[Merge to XFA] Revert "Revert "Add type cast definitions for CPDF_Dictionary.""

This reverts commit 937840e1722d1f2b77d80575d6e710d760662c9c.

Add type cast definitions for CPDF_Dictionary.

This CL adds ToCPDFDictionary type definitions and updates one file to use
instead of straight casts. I had to fix two places where we'd casted off the
constness of the original pointer.

BUG=pdfium:201
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1420583003 .

(cherry picked from commit 39869b641511c882d78e17548293cdb458c36f38)

Review URL: https://codereview.chromium.org/1410343003 .
35 files changed