Move enable extensions define to a build flag.

extensions/renderer/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")
 
 assert(enable_extensions)
 
 source_set("renderer") {
   sources = [
     "activity_log_converter_strategy.cc",
     "activity_log_converter_strategy.h",
     "api/automation/automation_api_helper.cc",
     "api/automation/automation_api_helper.h",
     "api/display_source/display_source_session.cc",
@@ skipping 295 matching lines @@
     "//ui/base",
   ]
 
   if (enable_wifi_display && proprietary_codecs) {
     sources += [
       "api/display_source/wifi_display/wifi_display_elementary_stream_descriptor_unittest.cc",
       "api/display_source/wifi_display/wifi_display_media_packetizer_unittest.cc",
     ]
   }
 }