Merge to XFA: Make (and verify) public/ files compile under C.

The following files needed adjustment beyond a mechanical merge:
   fpdfsdk/src/fpdfview.cpp
   public/fpdf_formfill.h
   samples/pdfium_test.cc
   testing/embedder_test.cpp

Original Review URL: https://codereview.chromium.org/1130843003

BUG=pdfium:158
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1137233002
15 files changed