GN: Tweak chrome_public_apk_tmpl to allow extra proguard_configs

chrome/android/chrome_public_apk_tmpl.gni

 # 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/config/android/rules.gni")
 
 declare_args() {
   # Whether chrome_public_apk should use the crazy linker.
   chrome_public_apk_use_chromium_linker = true
 
   # Whether chrome_public_apk should use the relocation packer.
   chrome_public_apk_use_relocation_packer = true
 
   # Whether native libraries should be loaded from within the apk.
   chrome_public_apk_load_library_from_apk = true
 }
 
 # GYP: //chrome/android/chrome_apk.gypi
 template("chrome_public_apk_tmpl") {
-  forward_variables_from(invoker, "*")
-
   android_apk(target_name) {
+    forward_variables_from(invoker, "*")
     _native_lib_file =
         rebase_path("$root_gen_dir/CHROME_VERSION.json", root_out_dir)
     native_lib_version_arg = "@FileArg($_native_lib_file:full-quoted)"
 
     if (!is_debug) {
       proguard_enabled = true
-      proguard_configs = [ "//chrome/android/java/proguard.flags" ]
+      _prev_proguard_configs = []
+      if (defined(proguard_configs)) {
+        _prev_proguard_configs = proguard_configs
+      }
+      proguard_configs = []
+      proguard_configs =
+          [ "//chrome/android/java/proguard.flags" ] + _prev_proguard_configs
     }
 
     if (chromium_linker_supported) {
       use_chromium_linker = chrome_public_apk_use_chromium_linker
 
       # TODO: Enable packed relocations for x64. See: b/20532404
       if (current_cpu != "x64") {
         enable_relocation_packing = chrome_public_apk_use_relocation_packer
       }
     }
   }
 }