Prevent base::CheckedNumeric from leaking outside of pdfium.

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 472 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 327 matching lines @@
     "testing/fx_string_testhelpers.h",
   ]
   deps = [
     "//testing/gtest",
     ":pdfium"
   ]
   include_dirs = [ "." ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ ":pdfium_config", "//build/config/compiler:no_chromium_code" ]
 }