Make (and verify) public/ files compile under C.

pdfium.gyp

 {
   'variables': {
     'pdf_use_skia%': 0,
     'conditions': [
       ['OS=="linux"', {
         'bundle_freetype%': 0,
       }, {  # On Android there's no system FreeType. On Windows and Mac, only a
             # few methods are used from it.
         'bundle_freetype%': 1,
       }],    
@@ skipping 846 matching lines @@
         '<(DEPTH)'
       ],
       'sources': [
         'core/src/fpdfapi/fpdf_parser/fpdf_parser_decode_embeddertest.cpp',
         'core/src/fpdfapi/fpdf_parser/fpdf_parser_parser_embeddertest.cpp',
         'fpdfsdk/src/fpdf_dataavail_embeddertest.cpp',
         'fpdfsdk/src/fpdfdoc_embeddertest.cpp',
         'fpdfsdk/src/fpdfformfill_embeddertest.cpp',
         'fpdfsdk/src/fpdftext_embeddertest.cpp',
         'fpdfsdk/src/fpdfview_embeddertest.cpp',
+        'testing/c_api_test.h',
+        'testing/c_api_test.c',
         'testing/embedder_test.cpp',
         'testing/embedder_test.h',
         'testing/embedder_test_mock_delegate.h',
         'testing/fx_string_testhelpers.cpp',
         'testing/fx_string_testhelpers.h',
       ],
     },
   ],
 }