Make the WARN_UNUSED_RESULT macro match the Chromium one.
Make the WARN_UNUSED_RESULT macro match the Chromium one.

BUG=none

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

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10259}

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