Some users aren't getting Software WebRender on nightly because they have things preventing them from getting (Hardware) WebRender, such as ANGLE support. We really should decouple these configurations.

Lack of support of (hardware) WebRender should not block Software
WebRender support. This can happen when ANGLE isn't supported, for
example.

Backed out changeset 465526830144 (Bug 1684170) for causing talos failures.

Backout link: https://hg.mozilla.org/integration/autoland/rev/555a05fe5330511dd715fdd6a39dc9d55baacd91

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&fromchange=1833e81ad2507cc66d4d431f9837239c6740a8e0&searchStr=windows%2C10%2Cx64%2Cwebrender%2Cshippable%2Copt%2Ctalos%2Cperformance%2Ctests%2Cwith%2Csoftware%2Cwebrender%2Cenabled%2Ctest-windows10-64-shippable-qr%2Fopt-talos-xperf-swr-e10s%2Cx&tochange=555a05fe5330511dd715fdd6a39dc9d55baacd91

Failure log: https://treeherder.mozilla.org/logviewer?job_id=325376070&repo=autoland&lineNumber=1140

Test now passes due to bug 1679215, although I don't believe the issues encountered were actually fixed, just papered over due to the temp scratch buffer we end up creating to work around the issue.

https://hg.mozilla.org/mozilla-central/rev/ebc2ea295bd6