This will make it easier to substitute in the future.

Bug: angleproject:5736
Change-Id: I0686c4c396e9f12cfb71253ccd9aa491a7942826
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2804812Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>