Move enable extensions define to a build flag.

chrome/browser/extensions/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("//chrome/common/features.gni")
+import("//extensions/features/features.gni")
 
 assert(enable_extensions)
 
 static_library("extensions") {
   sources = [
     # TODO These references to files in //chrome/browser/ should either be
     # moved to the browser target or the files moved to this directory.
     "../apps/app_launch_for_metro_restart_win.cc",
     "../apps/app_launch_for_metro_restart_win.h",
     "../apps/app_url_redirector.cc",
@@ skipping 862 matching lines @@
     "//components/zoom",
     "//content/app/resources",
     "//content/public/common",
     "//crypto",
     "//device/bluetooth",
     "//device/hid",
     "//extensions:extensions_resources",
     "//extensions/browser",
     "//extensions/browser/api:api_registration",
     "//extensions/common/api",
+    "//extensions/features",
     "//extensions/strings",
     "//net",
     "//printing/features",
     "//skia",
     "//sql",
     "//storage/browser",
     "//storage/common",
     "//third_party/WebKit/public:image_resources",
     "//third_party/WebKit/public:resources",
     "//third_party/cacheinvalidation",
@@ skipping 277 matching lines @@
       "api/braille_display_private/brlapi_connection.cc",
       "api/braille_display_private/brlapi_connection.h",
       "api/braille_display_private/brlapi_keycode_map.cc",
       "api/braille_display_private/brlapi_keycode_map.h",
     ]
     deps += [ "//build/linux/libbrlapi" ]
   } else {
     sources += [ "api/braille_display_private/braille_controller_stub.cc" ]
   }
 }