Remove enable_notifications build flag and define

ui/message_center/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("//testing/test.gni")
 
 # TODO(msw|mukai|dewittj): Move ash-specific files: crbug.com/585175
 component("message_center") {
   deps = [
     "//base",
     "//ui/base",
     "//url",
   ]
 
   defines = [ "MESSAGE_CENTER_IMPLEMENTATION" ]
 
-  if (enable_notifications && !is_android) {
+  if (!is_ios && !is_android) {
     deps += [
       "//base:i18n",
       "//base/third_party/dynamic_annotations",
       "//components/url_formatter",
       "//skia",
       "//ui/accessibility",
       "//ui/display",
       "//ui/events",
       "//ui/gfx",
       "//ui/gfx/geometry",
@@ skipping 130 matching lines @@
         "notifier_settings.cc",
         "notifier_settings.h",
       ]
     }
   }
 }
 
 static_library("test_support") {
   testonly = true
 
-  if (enable_notifications && !is_android) {
+  if (!is_ios && !is_android) {
     sources = [
       "fake_message_center.cc",
       "fake_message_center.h",
       "fake_message_center_tray_delegate.cc",
       "fake_message_center_tray_delegate.h",
       "fake_notifier_settings_provider.cc",
       "fake_notifier_settings_provider.h",
     ]
     deps = [
       "//base",
@@ skipping 30 matching lines @@
     "//ui/resources",
     "//ui/resources:ui_test_pak",
     "//url",
   ]
 
   data_deps = [
     "//third_party/mesa:osmesa",
     "//ui/resources:ui_test_pak_data",
   ]
 
-  if (enable_notifications && !is_android) {
+  if (!is_ios && !is_android) {
     sources += [
       "cocoa/notification_controller_unittest.mm",
       "cocoa/popup_collection_unittest.mm",
       "cocoa/popup_controller_unittest.mm",
       "message_center_impl_unittest.cc",
       "message_center_tray_unittest.cc",
       "notification_delegate_unittest.cc",
       "notification_list_unittest.cc",
     ]
 
@@ skipping 21 matching lines @@
     }
 
     if (!is_ios) {
       sources += [ "mojo/struct_traits_unittest.cc" ]
       deps += [
         "//mojo/edk/system",
         "//ui/gfx:test_support",
         "//ui/message_center/mojo:test_interfaces",
       ]
     }
-  }  # enable_notifications && !is_android
+  }  # !is_ios && !is_android
 }