Add android_libcpp_lib_dir gn arg

build/config/android/config.gni

Index: build/config/android/config.gni
diff --git a/build/config/android/config.gni b/build/config/android/config.gni
index 1009801ce2febfc7da0095bf584a57af4cd9914b..16d5373fa99188631e3bbe590735a4914735c0f5 100644
--- a/build/config/android/config.gni
+++ b/build/config/android/config.gni
@@ -50,6 +50,9 @@ if (is_android) {
     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"
 
@@ -257,4 +260,8 @@ if (is_android) {
   } else {
     assert(false, "Unknown Android ABI: " + current_cpu)
   }
+
+  if (android_libcpp_lib_dir == "") {
+    android_libcpp_lib_dir = "${android_libcpp_root}/libs/${android_app_abi}"
+  }
 }