Re: [WebCryptoAPI-tests] Added [SecureContext] to the subtle attribute (#5730)

Results are reproducible on Firefox nightly: 
https://travis-ci.org/w3c/web-platform-tests/jobs/228065029

But I'm unclear what's causing the flakiness. It seems it's hitting an assertion inside Firefox itself:

```
PROCESS | 5936 | (firefox:5936): GLib-GObject-CRITICAL **: g_object_ref: assertion 'object->ref_count > 0' failed
```

View on GitHub: https://github.com/w3c/web-platform-tests/pull/5730#issuecomment-298739494

Received on Tuesday, 2 May 2017 19:41:47 UTC