Allow compiling PDFium without V8.

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
   # methods are used from it.
   pdfium_bundle_freetype = !is_linux
 }
 
 config("pdfium_config") {
   cflags = []
   include_dirs = [ "third_party/freetype/include" ]
   defines = [
     "OPJ_STATIC",
+    "PDF_ENABLE_V8",
     "V8_DEPRECATION_WARNINGS",
     "_CRT_SECURE_NO_WARNINGS",
   ]
 
   if (pdf_use_skia) {
     defines += [ "_SKIA_SUPPORT_" ]
   }
 
   if (is_linux) {
     if (current_cpu == "x64") {
@@ skipping 767 matching lines @@
     "//v8",
     "//v8/include",
     "."
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }