This adds a WebRenderScrollData class (which contains a list of
WebRenderLayerScrollData objects, among other things), and adds it to
the DPEnd/DPSyncEnd messages sent across PWebRenderBridge. These classes are
skeletons for now (more stuff will be added to them in future patches).
MozReview-Commit-ID: 9duxwlUpdu7
Kartikaya Gupta