Bug 966397. Don't try to add fixed-pos metadata for inactive layer subtrees. r=mattwoodrow

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extra : rebase_source : 119635c63af471a0d6384901d99f242b14a39e9a
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

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extra : rebase_source : cfb56bb2111abce99cd475b9fa8c516d53621ed4