The test relies on resource priorities. Those don't roundtrip when serializing via NSURLRequest on Mavericks due to ResourceRequest::resourcePrioritiesEnabled() returning false.
It is bit surprising it works on release but not on debug. I suspect some debug code forces NSURLRequest construction and so we end up on the roundtrip code path.
Skipped on Maveriks: http://trac.webkit.org/changeset/182181
Removing Mavericks-specific lines from platform/mac/TestExpecations due to no longer supporting Mavericks.
Committed r193663: <http://trac.webkit.org/changeset/193663>