Add android_libcpp_lib_dir gn arg

build/config/android/config.gni

 # 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.
 
 # This file contains common system config stuff for the Android build.
 
 if (is_android) {
   assert(rebase_path("//", root_build_dir) == "../../",
          "Android output directory must be nested 2 levels within src/ (" +
              "e.g.: out-gn/Debug). http://crbug.com/412935")
@@ skipping 32 matching lines @@
   if (!defined(webview_framework_jar)) {
     webview_framework_jar =
         "//third_party/android_platform/webview/frameworks_6.0.jar"
   }
 
   declare_args() {
     android_sdk_root = default_android_sdk_root
     android_sdk_version = default_android_sdk_version
     android_sdk_build_tools_version = default_android_sdk_build_tools_version
 
+    # Libc++ library directory. Override to use a custom libc++ binary.
+    android_libcpp_lib_dir = ""
+
     # Android versionCode for android_apk()s that don't expclitly set one.
     android_default_version_code = "1"
 
     # Android versionName for android_apk()s that don't expclitly set one.
     android_default_version_name = "Developer Build"
 
     # The path to the keystore to use for signing builds.
     android_keystore_path = default_android_keystore_path
 
     # The name of the keystore to use for signing builds.
@@ skipping 187 matching lines @@
     android_app_secondary_abi = "mips64"
   } else if (current_cpu == "x64") {
     android_app_abi = "x86_64"
   } else if (current_cpu == "arm64") {
     android_app_abi = "arm64-v8a"
   } else if (current_cpu == "mips64el") {
     android_app_abi = "mips64"
   } else {
     assert(false, "Unknown Android ABI: " + current_cpu)
   }
+
+  if (android_libcpp_lib_dir == "") {
+    android_libcpp_lib_dir = "${android_libcpp_root}/libs/${android_app_abi}"
+  }
 }