[ios] Adds a buildflag for the ios_app_bundle_id_prefix.

ios/chrome/common/app_group/BUILD.gn

 # Copyright 2015 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/ios/ios_sdk.gni")
 
 source_set("app_group") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "app_group_constants.h",
     "app_group_constants.mm",
     "app_group_metrics.h",
     "app_group_metrics.mm",
   ]
 
   deps = [
     "//base",
     "//components/version_info",
+    "//ios/chrome/common:ios_app_bundle_id_prefix_header",
   ]
-
-  defines = [ "IOS_BUNDLE_ID_PREFIX=\"$ios_app_bundle_id_prefix\"" ]
 }
 
 # This target will be included into application extensions and the list
 # of its dependencies must be kept as short as possible.
 source_set("client") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "app_group_metrics_client.h",
     "app_group_metrics_client.mm",
   ]
 
   deps = [
     ":app_group",
     "//base",
   ]
 }
 
 source_set("main_app") {
   sources = [
     "app_group_metrics_mainapp.h",
     "app_group_metrics_mainapp.mm",
   ]
 
   deps = [
     ":app_group",
     "//base",
   ]
 }