Bug 876542. Rework TransformFixedLayers so that it explicitly makes layer anchor points invariant under a change in transform of a specified ancestor layer. r=Cwiiis,kats
Bug 876542 - Restore async fixed layer margin animation. r=roc, kats

Restore the reconciliation between content document fixed position margins at
render time and the fixed layer margins as retrieved via SyncViewportInfo
during async panning/zooming.
Bug 876542 - Restore async pan/zoom compensation behaviour during overscroll. r=roc

Bug 876542 introduces a patch that reworks how we maintain the position of
fixed position layers, but didn't handle overscroll/zoom. This restores the old
behaviour in this situation.
Bug 886298 - Convert Layers fixed position anchor to typed units. r=kats
Bug 886298 - Add a typed-units Margin. r=kats
Bug 886298 - Convert Layers fixed position margins to typed units. r=kats