Make the WARN_UNUSED_RESULT macro match the Chromium one.

BUG=none

Review URL: https://codereview.webrtc.org/1399313002

Cr-Commit-Position: refs/heads/master@{#10259}
diff --git a/webrtc/base/common.h b/webrtc/base/common.h
index e39b75d..e615c76 100644
--- a/webrtc/base/common.h
+++ b/webrtc/base/common.h
@@ -190,7 +190,7 @@
 // TODO(ajm): Hack to avoid multiple definitions until the base/ of webrtc and
 // libjingle are merged.
 #if !defined(WARN_UNUSED_RESULT)
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #define WARN_UNUSED_RESULT __attribute__((warn_unused_result))
 #else
 #define WARN_UNUSED_RESULT