Enable the -Wundef warning for clang

NOPRESUBMIT=true
BUG=webrtc:6398

Review-Url: https://codereview.webrtc.org/2358993004
Cr-Commit-Position: refs/heads/master@{#14425}
diff --git a/webrtc/modules/video_coding/decoding_state_unittest.cc b/webrtc/modules/video_coding/decoding_state_unittest.cc
index 3942dab..5d9733c 100644
--- a/webrtc/modules/video_coding/decoding_state_unittest.cc
+++ b/webrtc/modules/video_coding/decoding_state_unittest.cc
@@ -10,7 +10,7 @@
 
 #include <string.h>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/modules/video_coding/decoding_state.h"
 #include "webrtc/modules/video_coding/frame_buffer.h"