[Flake] grpc_ios_binary_size

```
Wed Aug  5 11:22:19 PDT 2020 - /Volumes/BuildData/tmpfs/src/github/grpc/src/objective-c/examples/Sample/Pods/Headers/Public/gRPC-Core/grpc/grpc.h:29:10: fatal error: 'grpc/impl/codegen/propagation_bits.h' file not found
```

https://source.cloud.google.com/results/invocations/c16be152-6a94-49b5-aaac-7930691d567a/targets/grpc%2Fcore%2Fpull_request%2Fmacos%2Fgrpc_ios_binary_size/log