Move enable extensions define to a build flag.

chrome/browser/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("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
 import("//components/os_crypt/features.gni")
+import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//net/features.gni")
 import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
 import("//third_party/protobuf/proto_library.gni")
 
 # //build/config/android/rules.gni imports //tools/grit/grit_rule.gni, which
 # produces a conflict for the "grit" template so we have to only include one.
 if (is_android) {
   import("//build/config/android/rules.gni")
@@ skipping 1401 matching lines @@
     "//content/public/common:feature_h264_with_openh264_ffmpeg",
     "//content/public/common:features",
     "//courgette:courgette_lib",
     "//crypto",
     "//crypto:platform",
     "//device/base",
     "//device/bluetooth:mojo",
     "//device/power_save_blocker",
     "//device/usb/mojo",
     "//device/usb/public/interfaces",
+    "//extensions/features",
     "//google_apis",
     "//gpu/config",
     "//media",
     "//media/midi",
     "//mojo/common",
     "//mojo/edk/system",
     "//mojo/public/cpp/bindings",
     "//mojo/public/js",
     "//net:extras",
     "//net:net_with_v8",
@@ skipping 2955 matching lines @@
   # linking all of the test support.
   static_library("pepper_cdm_test_constants") {
     testonly = true
     visibility = [ "//chrome/*" ]
     sources = [
       "media/pepper_cdm_test_constants.cc",
       "media/pepper_cdm_test_constants.h",
     ]
   }
 }