Per https://github.com/w3c/csswg-drafts/issues/2149 we should throw on invalid pseudo-elements. Let's try this behavior on Nightly.

This will make implementing the new behavior behind a pref
really straight-forward, and is generally nicer.

I need to update tests, try incoming.

Depends on D121705

With the patch above, some devtools tests failed because we were trying
to call getComputedStyle(node, ":marker") (read: one colon rather than
two).

Using two colons for pseudo-elements is the right thing to do and fixes
it / removes some weird special-cases.

Backed out 2 changesets (bug 1723921) for Mochitest failures in layout/inspector/tests/test_getCSSPseudoElementNames.html. CLOSED TREE

Log:
https://treeherder.mozilla.org/logviewer?job_id=347536827&repo=autoland&lineNumber=2327

https://treeherder.mozilla.org/logviewer?job_id=347536605&repo=autoland&lineNumber=1986

Push with failures:
https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&revision=e55c2177bbd2b98bd8005a7e5423bdae165ffaef

Backout:
https://hg.mozilla.org/integration/autoland/rev/0c7d036ab79c4bb37ad3c38ae5232c47e209eb84

Yeah, this depends on the other patch to work.

https://hg.mozilla.org/mozilla-central/rev/4c6d6481a449
https://hg.mozilla.org/mozilla-central/rev/a3bbc3330c2e
https://hg.mozilla.org/mozilla-central/rev/d9d140e0e045