Move enable extensions define to a build flag.

extensions/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/features.gni")
+import("//extensions/features/features.gni")
 
 source_set("browser") {
   sources = []
 
   deps = [
     "//base:i18n",
     "//components/cast_certificate",
     "//components/guest_view/browser",
     "//components/keyed_service/content",
     "//components/keyed_service/core",
     "//components/pref_registry",
     "//components/sessions",
     "//components/update_client",
     "//components/version_info",
     "//components/web_cache/browser",
     "//components/web_modal",
     "//components/zoom",
     "//content/public/browser",
     "//crypto:platform",
     "//extensions/browser/api:api_registration",
     "//extensions/common",
     "//extensions/common/api",
+    "//extensions/features",
     "//extensions/strings",
     "//google_apis",
     "//ui/display",
   ]
 
   configs += [
     "//build/config:precompiled_headers",
 
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
@@ skipping 391 matching lines @@
     "//components/update_client",
     "//components/url_matcher",
     "//components/user_prefs",
     "//content/test:test_support",
     "//device/bluetooth:mocks",
     "//device/power_save_blocker",
     "//extensions:extensions_browser_resources",
     "//extensions:test_support",
     "//extensions/common",
     "//extensions/common/api",
+    "//extensions/features",
     "//ipc:test_support",
     "//net:test_support",
     "//third_party/leveldatabase",
     "//third_party/zlib:zip",
   ]
 
   if (is_chromeos) {
     sources += [ "api/webcam_private/visca_webcam_unittest.cc" ]
     deps += [ "//chromeos:test_support" ]
   }
 }