Move enable extensions define to a build flag.

extensions/shell/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("//build/util/process_version.gni")
+import("//extensions/features/features.gni")
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 
 assert(enable_extensions)
 
 grit("resources") {
   source = "app_shell_resources.grd"
   outputs = [
     "grit/app_shell_resources.h",
     "app_shell_resources.pak",
@@ skipping 330 matching lines @@
     "//extensions/browser",
   ]
 
   if (is_chromeos) {
     deps += [
       "//chromeos:test_support",
       "//components/keyed_service/content",
     ]
   }
 }