[ios] Adds a buildflag for the ios_app_bundle_id_prefix.

ios/chrome/common/BUILD.gn

Index: ios/chrome/common/BUILD.gn
diff --git a/ios/chrome/common/BUILD.gn b/ios/chrome/common/BUILD.gn
index b3e6bed52d0f1729e48298ad70a0a2af40854e77..7b4a00f1210be8e916b7e709e6550e30fa8ec938 100644
--- a/ios/chrome/common/BUILD.gn
+++ b/ios/chrome/common/BUILD.gn
@@ -2,6 +2,9 @@
 # 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",
@@ -40,3 +43,8 @@ source_set("unit_tests") {
     "//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\"" ]
+}