Bug 1086937 patch 0 - Add missing null check of root element so this patch series doesn't expose a crash in layout/style/crashtests/472237-1.html .  r=birtles
Bug 1086937 patch 1 - Add eRestyle_ChangeAnimationPhaseDescendants restyle hint that is like eRestyle_ChangeAnimationPhase, but for a whole subtree.  r=birtles
Bug 1086937 patch 2 - Use eRestyle_ChangeAnimationPhaseDescendants to get the right style data in RestyleManager::RebuildAllStyleData.  r=birtles

Until we get rid of animation phases in bug 960465, we need to ensure
we're producing style data for the correct animation phase.  This makes
this optimization slightly less beneficial until then.
Bug 1086937 patch 3 - Add test for animations continuing across a user font set update.  r=birtles

I confirmed that without patch 2, the third and fourth tests fail
(reporting -1000px), whereas with the patches all 4 tests pass.