Update WARN_UNUSED_RESULT macro to match Chromium's version.
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-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10998}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc