Move enable extensions define to a build flag.

build/config/features.gni

 # 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.
 
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # These flags are effectively global. Your feature flag should go near the
 # code it controls. Most of these items are here now because they control
 # legacy global #defines passed to the compiler (now replaced with generated
 # buildflag headers -- see //build/buildflag_header.gni).
 #
 # There is more advice on where to put build flags in the "Build flag" section
 # of //build/config/BUILDCONFIG.gn.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 declare_args() {
-  enable_extensions = !is_android && !is_ios
   enable_plugins = (!is_android && !is_ios) || is_chromecast
   enable_pdf = !is_android && !is_ios && !is_chromecast
 
   # Enables Native Client support.
   # Temporarily disable nacl on arm64 linux to get rid of compilation errors.
   # TODO(mcgrathr): When mipsel-nacl-clang is available, drop the exclusion.
   enable_nacl = !is_ios && !is_android && !is_chromecast &&
                 current_cpu != "mipsel" && !(is_linux && target_cpu == "arm64")
 
   # Non-SFI is not yet supported on mipsel
@@ skipping 100 matching lines @@
 
 enable_configuration_policy = !is_ios
 
 enable_mac_keystone = is_mac && is_chrome_branded && is_official_build
 #
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # See comment at the top.