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

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

Committed: https://pdfium.googlesource.com/pdfium/+/cf22eb8ac24d944aac08d6a17c12b8a3a602c6f5

[Tom Sepez, 2015-05-12 23:30:28]

Lei, Not TBR'd, needs add'l review.

[Lei Zhang, 2015-05-13 00:09:06]

The list in fpdfview_c_api_test.c needs to be updated, right?

[Tom Sepez, 2015-05-13 00:26:44]

On 2015/05/13 00:09:06, Lei Zhang wrote:
> The list in fpdfview_c_api_test.c needs to be updated, right?

Done.

[Lei Zhang, 2015-05-13 00:27:34]

lgtm

[Tom Sepez, 2015-05-13 00:28:12]

Committed patchset #2 (id:20001) manually as
cf22eb8ac24d944aac08d6a17c12b8a3a602c6f5 (presubmit successful).