This is infrequent, so I haven't disabled the tests.

e.g. https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win7%20%28dbg%29/builds/10555
http/tests/security/contentSecurityPolicy/register-bypassing-scheme-partial.html

The issue reported here is that a thread had been created by the time the SchemeRegistry was modified (by calls via |internals|). These tests violate the assumption there that this is only changed during startup.

Either these tests should be changed, or they should be somehow exempt from this DCHECK.

I also suspect that these tests leave the state of the content_shell hosting them modified (because I don't see a mechanism for resetting the SchemeRegistry), but I haven't investigated.