Reland "Remove SPIRV_METAL references from .gn and tests" The backend was removed but the references were not. Update ShaderBinaryTest to properly skip tests if shader binaries are unsupported in the current ANGLE backend. Forcibly re-enable building of the Vulkan backend on macOS to keep ANGLE's SwiftShader backend working. Fixed: angleproject:6081 Change-Id: I5e6e47d5fe05b0dd6ec150b6db9fe5d75e580173 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4594582 Commit-Queue: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>