Previously we relied on a batch file, which for obvious reasons
isn't cross-platform.

BUG=angle:529

Change-Id: Ia1e3944f8ed2096773e68c39d48ae2dd7370897b
Reviewed-on: https://chromium-review.googlesource.com/186974Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>