http/tests/inspector/extensions-resources-redirect.html seems to sometimes cause the web process to crash. When this happens, the UI process then crashes in DrawingAreaProxyImpl::visibilityDidChange. See the URL for the crash log.
Bug 57030 covers the cause of the web process crash. The crashing test has now been added to the mac-wk2 Skipped file. But clearly there's still a UI process bug here.
Anders says this was fixed in r83509.
*** This bug has been marked as a duplicate of bug 58271 ***