Small fix to progress Manhattan MEC.

Bug: angleproject:4091
Test: Working through Manhattan frames 5-10, TRex still works
Change-Id: I7981e19a64f121b2ecca4451574ca2886641c6ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2039532
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>