Disable custommesh_cs_uniforms on Pixel3a (due to crash)

Change-Id: I508ba2d8640945904206d7f6e24e71cf6f45a5af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/864876
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>