[Android] Enable multidex for release builds of chrome_public_test_apk.

build/android/multidex.flags

+# 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 to keep through proguarding when multidex is
+# enabled. Not used during main dex list determination.
+
+-keepattributes *Annotations*
+-keep @interface org.chromium.base.annotations.MainDex
+-keep @**.MainDex class * {

[agrieve, 2017/04/23 01:47:52]

Not sure about this one. Why would we want to have @MainDex cause code to be not
stripped?

[jbudorick, 2017/04/24 15:05:00]

If we're trying to retain something in the main dex, presumably we want it to
not be stripped prior to dexing.

This file is only used when we're proguarding a multidex APK, so it will not
affect release builds of Chrome.

+  *;
+}