Masks Can No Longer Be Added to Locked Layers

This corrects unexpected behavior of being able to add masks to a locked layer even though
you could not edit the masks immediately after. Now, trying to add a mask to a locked layer
will not work.
BUG: 399145

(cherry picked from commit 8e19e4649445e6633a29e5586d923ede5cb0d4a4)