A compile error slipped in due to lack of test coverage.

Bug: angleproject:3611
Change-Id: Id34ce239f3913efb0ed1abab77321981419cab44
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1880160Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>