Move enable extensions define to a build flag.

BUILD.gn

 # Copyright (c) 2013 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 is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build_overrides/v8.gni")
+import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//remoting/remoting_enable.gni")
 import("//third_party/openh264/openh264_args.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
 import("//v8/gni/v8.gni")
 import("//v8/snapshot_toolchain.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
@@ skipping 1051 matching lines @@
   assert(target_name != "")  # Mark as used.
   sources = invoker.actual_sources
   assert(
       sources == invoker.actual_sources,
       "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
 }
 
 assert_valid_out_dir("_unused") {
   actual_sources = [ "$root_build_dir/foo" ]
 }