Currently, if you run a test that overrides a default web preference through layoutTestController, and the test crashes, the preference "WebKitDefaultPreferencesOverriden" persists on disk and gets re-loaded. This causes an assert when DRT starts up again. This value should be explicitly reset, so it does not cause an assert the next time that DRT runs. Also see bug 20534.
Bug 20534 addresses this issue in its patch, so I'm resolving this one as a duplicate. *** This bug has been marked as a duplicate of 20534 ***