Bug 907743 - Align display port to tile boundaries when tiles are enabled. r=botond,kats

This aligns display ports to tile boundaries, taking care to compensate for
the difference between the requested scroll position and the actual scroll
position.
Bug 943348 - Make sure displayport is set on before-first-paint in TabChild. r=botond

Make sure to set the scroll ID on the FrameMetrics used to update on the
before-first-paint signal.