Allow overriding GN build flags

build_overrides/pdfium.gni

-# Copyright 2015 PDFium Authors. All rights reserved.
+# Copyright 2016 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.
 
 # This file contains PDFium-related build flags.
 
 declare_args() {
   # On Android there's no system FreeType. On Windows and Mac, only a few
   # methods are used from it.
   pdfium_bundle_freetype = !is_linux
 
   # Build PDFium either with or without v8 support.
   pdf_enable_v8 = true
 
   # Build PDFium either with or without XFA Forms support.
   pdf_enable_xfa = false
 
   # Build PDFium against skia (experimental) rather than agg.
   pdf_use_skia = false
 
   # Build PDFium standalone
   pdf_is_standalone = false
 }