We don't currently have a mechanism for rerendering when the front/back
flips, so we should disable running such animations on the compositor
thread for now until we do.
Bug 1186204 covers reenabling.
The reftest fails without the patch (showing a blue almost-square
rectangle), and passes with the patch.
The use of reftest-no-flush (added in patch 1) is needed to achieve the
failure without the patch, because the flushWindow() function in
reftest-content.js calls getBoundingClientRect() to flush rendering,
which has the side-effect of flushing style updates that have been
suppressed on the main thread while we're running an animation off the
main thread, which in turn covers up the bug.
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