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

components/style/bloom.rs

@@ -122,6 +122,12 @@ impl<E: TElement> StyleBloom<E> {
         self.elements.is_empty()
     }
 
+    /// Returns the DOM depth of elements that can be correctly
+    /// matched against the bloom filter (that is, the number of
+    /// elements in our list).
+    pub fn matching_depth(&self) -> usize {
+        self.elements.len()
+    }
 
     /// Clears the bloom filter.
     pub fn clear(&mut self) {