Enforce WARN_UNUSED_RESULT attribute on Windows

BUG=none
R=cpu,danakj,dpranke,hans,kelvinp,rickyz,sky,sorin,thestig,tuttle,yukawa
TBR=vitalybuka

Review URL: https://codereview.chromium.org/1390223002

Cr-Commit-Position: refs/heads/master@{#353450}


CrOS-Libchrome-Original-Commit: 7764fe9ea52233096b7916614eb7c60b26bbca05
diff --git a/base/compiler_specific.h b/base/compiler_specific.h
index 02603df..d5d38b7 100644
--- a/base/compiler_specific.h
+++ b/base/compiler_specific.h
@@ -119,7 +119,9 @@
 // Use like:
 //   int foo() WARN_UNUSED_RESULT;
 // To explicitly ignore a result, see |ignore_result()| in base/macros.h.
-#if defined(COMPILER_GCC)
+// TODO(dcheng): Update //third_party/webrtc's macro definition to match.
+#undef WARN_UNUSED_RESULT
+#if defined(COMPILER_GCC) || defined(__clang__)
 #define WARN_UNUSED_RESULT __attribute__((warn_unused_result))
 #else
 #define WARN_UNUSED_RESULT