Allow overriding GN build flags

BUILD.gn

 # 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.
 
 import("//build_overrides/v8.gni")
+import("//build_overrides/pdfium.gni")
 import("//testing/test.gni")
-import("pdfium.gni")
 
 config("pdfium_config") {
   cflags = []
   include_dirs = [
     ".",
     "third_party/freetype/include",
     "third_party/freetype/include/freetype",
   ]
   defines = [
     "OPJ_STATIC",
@@ skipping 1690 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }