Backed out 6 changesets (bug 1456995) for devtools failure at browser/browser_aboutdebugging_serviceworker_timeout.js. On a CLOSED TREE

Backed out changeset 2237ceb903a4 (bug 1456995)
Backed out changeset 5ce169fa82fb (bug 1456995)
Backed out changeset e64553b959f9 (bug 1456995)
Backed out changeset 7c53d7ca5ef7 (bug 1456995)
Backed out changeset 67e23c511201 (bug 1456995)
Backed out changeset e127b732bab0 (bug 1456995)