Revert "Revert "Add type cast definitions for CPDF_Dictionary.""

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

Committed: https://pdfium.googlesource.com/pdfium/+/39869b641511c882d78e17548293cdb458c36f38

[dsinclair, 2015-10-20 18:49:33]

dsinclair@chromium.org changed reviewers:
+ thestig@chromium.org

[dsinclair, 2015-10-20 18:49:33]

PTAL.

PS1 is exactly the reverted CL. PS2 shows the changes to fix the crash.

The test crash was due to using AsDictionary instead of ToDictionary in
core/src/fpdfapi/fpdf_render/fpdf_render.cpp.

I changed one other AsDictionary as Clone can return nullptr.

[Tom Sepez, 2015-10-20 18:55:23]

lgtm

[dsinclair, 2015-10-20 18:57:59]

Committed patchset #2 (id:20001) manually as
39869b641511c882d78e17548293cdb458c36f38 (presubmit successful).