Extern in .cpp files is a code smell.

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 299 matching lines @@
       'ldflags': [ '-L<(PRODUCT_DIR)',],
       'sources': [
         'core/include/fpdftext/fpdf_text.h',
         'core/src/fpdftext/fpdf_text.cpp',
         'core/src/fpdftext/fpdf_text_int.cpp',
         'core/src/fpdftext/fpdf_text_search.cpp',
         'core/src/fpdftext/text_int.h',
         'core/src/fpdftext/txtproc.h',
         'core/src/fpdftext/unicodenormalization.cpp',
         'core/src/fpdftext/unicodenormalizationdata.cpp',
+        'core/src/fpdftext/unicodenormalizationdata.h',
       ],
     },
     {
       'target_name': 'fxcodec',
       'type': 'static_library',
       'dependencies': [
         'third_party/third_party.gyp:fx_lcms2',
         'third_party/third_party.gyp:fx_libjpeg',
         'third_party/third_party.gyp:fx_libopenjpeg',
         'third_party/third_party.gyp:fx_zlib',
@@ skipping 134 matching lines @@
         'core/src/fxge/apple/apple_int.h',
         'core/src/fxge/apple/fx_apple_platform.cpp',
         'core/src/fxge/apple/fx_mac_imp.cpp',
         'core/src/fxge/apple/fx_quartz_device.cpp',
         'core/src/fxge/dib/dib_int.h',
         'core/src/fxge/dib/fx_dib_composite.cpp',
         'core/src/fxge/dib/fx_dib_convert.cpp',
         'core/src/fxge/dib/fx_dib_engine.cpp',
         'core/src/fxge/dib/fx_dib_main.cpp',
         'core/src/fxge/dib/fx_dib_transform.cpp',
+        'core/src/fxge/fontdata/chromefontdata/chromefontdata.h',
         'core/src/fxge/fontdata/chromefontdata/FoxitDingbats.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitFixed.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitFixedBold.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitFixedBoldItalic.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitFixedItalic.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitSans.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitSansBold.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitSansBoldItalic.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitSansItalic.c',
         'core/src/fxge/fontdata/chromefontdata/FoxitSansMM.c',
@@ skipping 270 matching lines @@
         'testing/embedder_test.cpp',
         'testing/embedder_test.h',
         'testing/embedder_test_mock_delegate.h',
         'testing/embedder_test_timer_handling_delegate.h',
         'testing/fx_string_testhelpers.cpp',
         'testing/fx_string_testhelpers.h',
       ],
     },
   ],
 }