Move enable extensions define to a build flag.

apps/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("//build/config/ui.gni")
+import("//extensions/features/features.gni")
 
 assert(!is_android && !is_ios)
 
 static_library("apps") {
   sources = []
 
   if (enable_extensions) {
     sources += [
       "app_lifetime_monitor.cc",
       "app_lifetime_monitor.h",
@@ skipping 46 matching lines @@
       "//ui/views",
     ]
     if (enable_extensions) {
       deps += [ "//extensions/browser" ]
     }
   }
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 }