Fix the watchOS build for the mirror clamp to edge extension The mirror clamp to edge extension relies on MTLSamplerAddressModeMirrorClampToEdge, which is not available on watchOS. Disable the extension there for the moment. Bug: angleproject:8103 Change-Id: I52b9d5217b39080cde6663e0d0fa839e53dacecf Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4379037 Reviewed-by: Kenneth Russell <kbr@chromium.org>