[Android] Add gn support for multidex.

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
 
@@ skipping 14 matching lines @@
 
     if (!is_debug) {
       proguard_enabled = true
       _prev_proguard_configs = []
       if (defined(proguard_configs)) {
         _prev_proguard_configs = proguard_configs
       }
       proguard_configs = []
       proguard_configs =
           [ "//chrome/android/java/proguard.flags" ] + _prev_proguard_configs
+    } else {
+      multidex = true

[agrieve, 2015/11/16 19:02:33]

super nit: can you flip the clauses so it reads "if (is_debug) {..."

[jbudorick, 2015/11/16 20:45:56]

Done.

     }
 
     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
       }
     }
   }
 }