Move enable extensions define to a build flag.

chrome/browser/ui/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/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
+import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//printing/features/features.gni")
 
 config("ui_warnings") {
   if (is_clang) {
     # TODO(thakis): Remove this once http://crbug.com/383820 is figured out
     cflags = [ "-Wno-nonnull" ]
   }
 }
 
@@ skipping 535 matching lines @@
     "//components/version_ui",
     "//components/web_cache/browser",
     "//components/web_resource",
     "//components/zoom",
     "//content/app/resources",
     "//content/public/common",
     "//crypto",
     "//device/base",
     "//device/bluetooth/public/interfaces:experimental_interfaces",
     "//device/usb",
+    "//extensions/features",
     "//media",
     "//net:net_with_v8",
     "//printing/features",
     "//services/service_manager/runner/common",
     "//skia",
     "//storage/browser",
     "//storage/common",
     "//third_party/WebKit/public:features",
     "//third_party/WebKit/public:resources",
     "//third_party/adobe/flash:flapper_version_h",
@@ skipping 2885 matching lines @@
       "test/test_confirm_bubble_model.cc",
       "test/test_confirm_bubble_model.h",
     ]
     deps += [ "//chrome/test:test_support_ui" ]
   }
 
   if (enable_extensions) {
     deps += [ "//extensions/browser" ]
   }
 }