Chromium builds define UNICODE, while we don't, which confuses
the built-in IDC_ARROW macro.

With this fix, and http://crbug.com/415983, we can build
angle_perf_tests in Chromium.

BUG=angle:744

Change-Id: I268a7c11a5b5355cb2485bf10faf750972cf4e30
Reviewed-on: https://chromium-review.googlesource.com/220363Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>