Once the fission pref flips defaults, we'll want to also update the harnesses to use fission by default. This will involve:

1. Removing this pref. Note we can move it into suite specific profiles if we want to migrate harnesses one at a time.
2. Updating task configurations such that we pass in --setpref=fission.autostart=false for non-fission tasks (rather than --setpref=fission.autostart=true for fission tasks).

The ideal way to solve #2 would be to remove the fission variant and replace it with a no-fission variant (name tbd). Though doing this means we'll be changing the labels, which we may want to wait and batch up all at once alongside e10s and qr as well.

So an easier way of solving this would be to leave fission as a variant, then add a temporary transform afterwards that sets --setpref=fission.autostart=false for any task that doesn't have "fission" in its variant name. It's a bit hackier, but should only be temporary and reduces the moving parts in this bug.

Note I believe there is a separate bug for WPT that :jgraham is working on, so we should ignore that suite here.

https://hg.mozilla.org/mozilla-central/rev/44031c20322e

Explicitly --setpref fission.autostart as needed whenever the task
fission-ness does not match the default.

Run with fission by default in test harnesses, with --disable-fission available as an
option.
Android mach commands automatically set --disable-fission; this can be over-ridden by
--setpref=fission.autostart=true.
fission.autostart is removed from all test profiles.

Backed out for causing Jit failures.

Backout link: https://hg.mozilla.org/integration/autoland/rev/57d47cc95fa38e56e7032db0ed22841d15071ccb

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=FDPblOWxSsKmAj64ov1zbQ.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=0c43fef8293a4324097cbf0d4e326e5e6b566b26

Failure log: https://treeherder.mozilla.org/logviewer?job_id=363405302&repo=autoland&lineNumber=1502

Thanks.

Should be easily addressed:

https://treeherder.mozilla.org/jobs?repo=try&revision=0f63fff9ac06dcbea4585157641dcf778d96856d

https://hg.mozilla.org/mozilla-central/rev/7c7e9ff61409
https://hg.mozilla.org/mozilla-central/rev/b3d3ea2b3683