Auto merge of #17968 - emilio:no-sharing-if-not-restyled, r=bzbarsky
Auto merge of #17968 - emilio:no-sharing-if-not-restyled, r=bzbarsky

style: Prevent cousin sharing if we haven't restyled the parents.

This "fixes" bug 1381821 by disabling the optimization in the cases we can't
prove this is correct. Bug 1387116 tracks fixing the optimization to apply more
broadly.

Bug: 1381821
Reviewed-by: bz
MozReview-Commit-ID: COKdmHHokGx

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