Replace uses of MUST_USE_RESULT by V8_WARN_UNUSED_RESULT in ElapsedTimer.

R=bmeurer@chromium.org
Signed-off-by: Thiago Farina <tfarina@chromium.org>

Review URL: https://codereview.chromium.org/23523034

Patch from Thiago Farina <tfarina@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16583 ce2b1a6d-e550-0410-aec6-3dcde31c8c00