[ios] Adds a buildflag for the ios_app_bundle_id_prefix.

ios/chrome/common/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/buildflag_header.gni")
+import("//build/config/ios/ios_sdk.gni")
+
 source_set("common") {
   sources = [
     "channel_info.h",
     "channel_info.mm",
     "material_timing.h",
     "material_timing.mm",
     "string_util.h",
     "string_util.mm",
     "x_callback_url.cc",
     "x_callback_url.h",
@@ skipping 18 matching lines @@
     "ns_regular_expression_unittest.mm",
     "string_util_unittest.mm",
     "x_callback_url_unittest.cc",
   ]
   deps = [
     ":common",
     "//base",
     "//testing/gtest",
   ]
 }
+
+buildflag_header("ios_app_bundle_id_prefix_header") {
+  header = "ios_app_bundle_id_prefix.h"
+  flags = [ "IOS_APP_BUNDLE_ID_PREFIX=\"$ios_app_bundle_id_prefix\"" ]
+}