Add workaround for Unicode compile error. 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/220363 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>