Bug 844181 - Fix testing-impacting only failure in crashes provider; r=me, a=bustage + test-only + dolske

The code this is patching was green on mozilla-central a little over a
week ago and is now failing on Aurora for an unknown reason. There is a
legitimate bug and the fix is trivial.

The xpcshell failure this fixes was only occurring on Windows and
appeared to be intermittent (although there was only 1 success out of 8+
runs). The change only impacts test code because the added return
statement prevents a race condition that should only occur during tests.

After a discussion in #developers, it was decided that because of the
trivial nature of the fix and the alternative of backing out FHR would
be drastic, that it would be OK to push this without explicit approval
from one of the usual suspects.