Enable the -Wundef warning for clang

webrtc/modules/desktop_capture/rgba_color_unittest.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/desktop_capture/rgba_color.h"
 
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 
 TEST(RgbaColorTest, ConvertFromAndToUInt32) {
   static const std::vector<uint32_t> cases{
       0,         1000,       2693,       3725,       4097,      12532,
       19902,     27002,      27723,      30944,      65535,     65536,
       231194,    255985,     322871,     883798,     9585200,   12410056,
       12641940,  30496970,   105735668,  110117847,  482769275, 542368468,
       798173396, 2678656711, 3231043200, UINT32_MAX,
   };
 
   for (uint32_t value : cases) {
     RgbaColor left(value);
     ASSERT_EQ(left.ToUInt32(), value);
     RgbaColor right(left);
     ASSERT_EQ(left.ToUInt32(), right.ToUInt32());
   }
 }
 
 TEST(RgbaColorTest, AlphaChannelEquality) {
   RgbaColor left(10, 10, 10, 0);
   RgbaColor right(10, 10, 10, 255);
   ASSERT_EQ(left, right);
   right.alpha = 128;
   ASSERT_NE(left, right);
 }
 
 }  // namespace webrtc