Add support for proguard preprocessing.

third_party/guava/proguard.flags

+# Copyright 2013 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.
+
+-keep class com.google.common.base.** {
+  *;
+}
+
+-keep class com.google.common.annotations.** {
+  *;
+}
+
+-keep class com.google.common.collect.** {
+  *;
+}
+
+-keepattributes Signature
+
+# Don't complain about usage of sun.misc.Unsafe. Guava imports this,
+# but does not use it unless it exists.
+# The gyp-target that uses this is guava.gyp:guava_javalib.
+-dontwarn sun.misc.Unsafe
+# Striped64 uses reflection to access some local fields.
+-dontnote com.google.common.cache.Striped64
+-dontnote com.google.common.cache.Striped64$Cell
+
+# Keep all enum values and valueOf methods. See
+# http://proguard.sourceforge.net/index.html#manual/examples.html
+# for the reason for this. Also, see http://crbug.com/248037.
+-keepclassmembers enum * {
+    public static **[] values();
+    public static ** valueOf(java.lang.String);
+}