Bug 966397 - Part 1: Don't try to add fixed-pos metadata for inactive layer subtrees. r=mattwoodrow, a=bajaj
Bug 966397 - Part 2: Don't look at ancestors of mContainerReferenceFrame when searching for a fixed-pos ancestor we belong to, since we'll change coordinate spaces. The metadata will be attached to an ancestor layer in that case. r=mattwoodrow, a=bajaj