When destroying SurfaceMtl, the CAMetalLayer should detach from its
super layer, otherwise memory leak could happen.
This memory leak caused the end2end tests to ran out of memory.

Bug: angleproject:2634
Change-Id: I8ea91cccdb42cf0ae8ef807cf01f1e05043dc7c1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2060250
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>