Metal: Fixed memory leak due to the CAMetalLayer still attached to super layer 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>