Auto merge of #17172 - bzbarsky:cache-parent-level, r=emilio

Parent mismatch should not clear style sharing cache.

We can have cousins in the cache whose parent doesn't match ours, and other
cousins whose parent does.  When we encounter one of the former, that's not a
reason to stop lookin for the latter.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1369620

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