Remove some asserts that prevent Dawn from completing a DM run.

Dawn does not support readback from a bare render target, so it will
continued to fail those tests. But it should fail without dm halting
execution.

Change-Id: If7955106d5dbb4eaea86af58de96bcc7e74aac85
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/288902
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
3 files changed