[Android] Enable multidex for release builds of chrome_public_test_apk.

build/android/main_dex_classes.flags

Index: build/android/main_dex_classes.flags
diff --git a/build/android/main_dex_classes.flags b/build/android/main_dex_classes.flags
index 81152dcc1eab8c18aff179054e6eb7b8186e0348..9bb7977afe3fe98a3e6b9f47b575ceac780eb1ac 100644
--- a/build/android/main_dex_classes.flags
+++ b/build/android/main_dex_classes.flags
@@ -1,3 +1,10 @@
+# Copyright 2017 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.
+
+# Proguard flags for what should be kept in the main dex. Only used
+# during main dex list determination, not during actual proguarding.
+
 -keep @**.MainDex class * {
   *;
 }