Update WARN_UNUSED_RESULT macro to match Chromium's version.

It also matches the one in webrtc/base/common.h

BUG=None
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1516423004

Cr-Original-Commit-Position: refs/heads/master@{#10998}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 66679dcc04658e5e0dc334f53e647eb9eb3107ed
1 file changed