Allow overriding GN build flags

pdfium.gni

-# Copyright 2015 PDFium 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
-}