style: Don't pop too much in the bloom filter.

This is not a problem for Servo, because we recovered it without much problem a
few lines below, and we never reached the pathological case where the filter has
one element (the root), and we restyled another element with depth 1.

Fixes bug 1321725.

components/style/bloom.rs

@@ -181,7 +181,7 @@ impl StyleBloom {
 
         // If the filter represents an element too deep in the dom, we need to
         // pop ancestors.
-        while current_depth >= element_depth - 1 {
+        while current_depth > element_depth - 1 {
             self.pop::<E>().expect("Emilio is bad at math");
             current_depth -= 1;
         }