[ios] Adds a buildflag for the ios_app_bundle_id_prefix.

build/config/ios/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/buildflag_header.gni")
 import("//build/config/ios/ios_sdk.gni")
 import("//build/config/sysroot.gni")
 import("//build/toolchain/toolchain.gni")
 
 # This is included by reference in the //build/config/compiler:runtime_library
 # config that is applied to all targets. It is here to separate out the logic
 # that is iOS-only. Please see that target for advice on what should go in
 # :runtime_library vs. :compiler.
 config("runtime_library") {
   common_flags = [
@@ skipping 42 matching lines @@
 
   libs = [
     "Foundation.framework",
     "XCTest.framework",
   ]
 }
 
 group("xctest") {
   public_configs = [ ":xctest_config" ]
 }
+
+buildflag_header("ios_app_bundle_id_prefix_header") {

[rohitrao (ping after 24h), 2016/12/13 14:14:30]

Is this the right GN file for this header?

[sdefresne, 2016/12/13 15:18:02]

I think it would be better in ios/chrome/common (at some point I'd like to maybe
move the ios_app_bundle_prefix variable there too).

[rohitrao (ping after 24h), 2016/12/13 15:20:47]

So I'll leave the variable in ios_sdk.gni but I'll move the buildflag_header to
common?

+  header = "ios_app_bundle_id_prefix.h"
+  flags = [ "IOS_APP_BUNDLE_ID_PREFIX=\"$ios_app_bundle_id_prefix\"" ]
+}