Re-enable `flutter test --platform chrome` on Windows

[jonasfj]

In https://github.com/google/webcrypto.dart/pull/117 I disabled `flutter test --platform chrome' on Windows in order to make Github Actions tests pass. For the record this has been failing in many PRs, so it wasn't related to the specific PR in which it was disabled. It was just disabled as to avoid blocking development.

I don't know why they were failing, I think it's more to do with a bug in Flutter or maybe configuration of the machine.

I haven't tried to reproduce it else where, but we do run the same test cases with flutter pub run test -p chrome later, so it's not like we don't test it. It's just that the flutter test --platform chrome mechanism doesn't work.

We should find out if this is intentionally broken by Flutter, if this is a bug in Flutter, a bug in how our machine is configured, or some other form of bad luck :see_no_evil: