This was showing up in the following test for me on OSX:

IOSurfaceClientBufferTest.CopySubImageToBGRX8888IOSurface/ES2_Vulkan_SwiftShader

Test: angle_end2end_tests on OSX 10.15
Bug: angleproject:5147
Change-Id: I666dc832e8be33cde1617c849990c6092f4e2240
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2462041Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>