Metal: Disable prebuilt shaders on iOS. Chromium's Mac SDK is missing files needed to build for iOS: LLVM ERROR: Error opening '/Volumes/Work/s/w/ir/cache/xcode_ios_15a507.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/metal/macos/bin/../lib/clang/32023.35/lib/darwin/libmetal_rt_ios.a': No such file or directory! Bug: chromium:1385510 Change-Id: Iee7d6f477ffbebbbdca22e8377f244d2d13dfe66 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4980045 Reviewed-by: Roman Lavrov <romanl@google.com> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>