The comments bug 1533739 suggest that this was already fixed a few weeks ago with https://hg.mozilla.org/mozilla-central/rev/72c8227148dd in bug 1532863. Gyula Palko confirmed with mozregression here https://bugzilla.mozilla.org/show_bug.cgi?id=1533739#c12.
Bug 1542677 Comment 5 Edit History
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The comments in bug 1533739 suggest that this was already fixed a few weeks ago with https://hg.mozilla.org/mozilla-central/rev/72c8227148dd in bug 1532863. Gyula Palko confirmed with mozregression here https://bugzilla.mozilla.org/show_bug.cgi?id=1533739#c12.