Metal: Add fall back to from-source shader compilation We can't compile Metal shaders at build time without the chromium build directory and mac_sdk.gni. Add a graceful fallback to building the shaders from source at runtime in this case. Also detect the iOS simulator in gn and disable shader compilation. We don't have all the Mac SDK files needed to build shaders for this target yet. Bug: chromium:1385510 Change-Id: Ibd1b36f9d97e7340a9742cba0c9c5f2221cd9a8d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4974918 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>