Add support for proguard preprocessing.

third_party/guava/guava.gyp

 # Copyright (c) 2012 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.
 
 {
   'targets': [
     {
       'target_name': 'guava_javalib',
       'type': 'none',
       'variables': {
         'java_in_dir': 'src/guava',
+        'proguard_preprocess': 1,
+        'proguard_config': 'proguard.flags',
       },
       'dependencies': [
         '../../third_party/jsr-305/jsr-305.gyp:jsr_305_javalib',
       ],
       'includes': [ '../../build/java.gypi' ],
     },
   ],
 }