Bug 936500 - Always build layers for nsDisplayScrollLayer. r=tn

APZC information is associated with layers, and having those layers suddenly
disappear because they became occluded, or their displayports are off-screen
has undesired effects.
Bug 936500 - Don't copy Axis when creating a FlingAnimation. r=kats

FlingAnimation was, likely accidentally, creating copies of the axes it was
animating and operating on those, which lead to problems when displayports
were calculated based on the original copies which housed old values.