extensions: Check |enable_extensions| flag before depending on extension code.

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")
 
 static_library("apps") {
   sources = [
     "app_lifetime_monitor.cc",
@@ skipping 45 matching lines @@
     ]
     sources = []
   }
 
   if (toolkit_views) {
     sources += [
       "ui/views/app_window_frame_view.cc",
       "ui/views/app_window_frame_view.h",
     ]
     deps += [
-      "//extensions/browser",
       "//ui/strings",
       "//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" ]
 }