Prevent base::CheckedNumeric from leaking outside of pdfium.

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 498 matching lines @@
         'core/include/fxcrt/fx_basic.h',
         'core/include/fxcrt/fx_coordinates.h',
         'core/include/fxcrt/fx_ext.h',
         'core/include/fxcrt/fx_memory.h',
         'core/include/fxcrt/fx_stream.h',
         'core/include/fxcrt/fx_string.h',
         'core/include/fxcrt/fx_system.h',
         'core/include/fxcrt/fx_ucd.h',
         'core/include/fxcrt/fx_xml.h',
         'core/src/fxcrt/extension.h',
+        'core/src/fxcrt/fx_safe_types.h',
         'core/src/fxcrt/fxcrt_platforms.cpp',
         'core/src/fxcrt/fxcrt_platforms.h',
         'core/src/fxcrt/fxcrt_posix.cpp',
         'core/src/fxcrt/fxcrt_posix.h',
         'core/src/fxcrt/fxcrt_windows.cpp',
         'core/src/fxcrt/fxcrt_windows.h',
         'core/src/fxcrt/fx_arabic.cpp',
         'core/src/fxcrt/fx_arabic.h',
         'core/src/fxcrt/fx_basic_array.cpp',
         'core/src/fxcrt/fx_basic_bstring.cpp',
@@ skipping 330 matching lines @@
         'fpdfsdk/src/fpdftext_embeddertest.cpp',
         'fpdfsdk/src/fpdfview_embeddertest.cpp',
         'testing/embedder_test.cpp',
         'testing/embedder_test.h',
         'testing/fx_string_testhelpers.cpp',
         'testing/fx_string_testhelpers.h',
       ],
     },
   ],
 }