Add workaround for Unicode compile error.
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 <0f3694938fd5703bd548127ee497e4d604509bef@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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