Extern in .cpp files is a code smell.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 
 pdf_use_skia = false
 
 declare_args() {
   # On Android there's no system FreeType. On Windows and Mac, only a few
@@ skipping 305 matching lines @@
 static_library("fpdftext") {
   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",
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }
 
 static_library("fxcodec") {
   deps = [
@@ skipping 133 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 295 matching lines @@
     "//v8",
     "//v8/include",
     "."
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }