Replace all uses with V8_WARN_UNUSED_RESULT.
WARN_UNUSED_RESULT was defined in src/base/compiler-specific.h, which
includes include/v8config.h, which already defined
V8_WARN_UNUSED_RESULT.

R=mstarzinger@chromium.org

Bug: v8:7570
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I662072294605036ca5aa0c8fdaa0218ac5d95f23
Reviewed-on: https://chromium-review.googlesource.com/998893Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52457}