[Android] Enable multidex for release builds of chrome_public_test_apk.

build/android/multidex.flags

Index: build/android/multidex.flags
diff --git a/build/android/multidex.flags b/build/android/multidex.flags
new file mode 100644
index 0000000000000000000000000000000000000000..59e7e856ce50f81232b19d85a0fb00667f5308b4
--- /dev/null
+++ b/build/android/multidex.flags
@@ -0,0 +1,12 @@
+# 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 * {
+  *;
+}