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/base/bitbuffer_unittest.cc b/webrtc/base/bitbuffer_unittest.cc
index 7ba9c73..b06bf49 100644
--- a/webrtc/base/bitbuffer_unittest.cc
+++ b/webrtc/base/bitbuffer_unittest.cc
@@ -168,7 +168,7 @@
 
   // Disable death test on Android because it relies on fork() and doesn't play
   // nicely.
-#if defined(GTEST_HAS_DEATH_TEST)
+#if GTEST_HAS_DEATH_TEST
 #if !defined(WEBRTC_ANDROID)
   // Passing a NULL out parameter is death.
   EXPECT_DEATH(buffer.GetCurrentOffset(&byte_offset, NULL), "");