This bug was hit by a KHR ES 3.1 test.

This CL just applies the obvious fix already noted by jmadill@.

Test: KHR-GLES31.core.shader_image_size.*-nonMS-*
Bug: angleproject:4008
Bug: angleproject:4108
Change-Id: I5d05c0a7432f7b70d2b995fd8b11224c119ece5f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2042491Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>