Example failure:
https://ci.chromium.org/p/webrtc/builders/luci.webrtc.ci/iOS64%20Sim%20Debug%20%28iOS%209.0%29/7661

Recipe that raises the exception:
https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/ios/api.py?l=1131&rcl=fdd5011d0209a5a59eadeb77cc849aa2b22f3b4b

I'm pretty sure it does everything correctly by raising an InfraFailure but for some reason the build is marked as a normal failure.

This might be because the recipe engine does not detect InfraFailure but just sees it as a generic StepFailure:
https://cs.chromium.org/chromium/infra/recipes-py/recipe_engine/run.py?rcl=0c993cb828b335e2fc0528c149f8060c35ebf0ac&l=291