Avoid bloom filter churn.

When a cached bloom filter is found during traversal, there are two
cases, both of which currently do unnecessary allocations. This patch
avoids these allocations. In the process, it renders correct two
previously-incorrect comments, and moves one of those comments into a
better spot.

While scrolling moderately fast all the way through the "Guardians of
the Galaxy" Wikipedia page, this patch (a) avoids 1.2 million calls to
`clone()` and (b) replaces 111,000 `BloomFilter::new()` calls with
`clear()` calls.

components/layout/traversal.rs

@@ -73,18 +73,17 @@ fn take_task_local_bloom_filter(parent_node: Option<LayoutNode>, layout_context:
             }
             // Found cached bloom filter.
             (Some(parent), Some((mut bloom_filter, old_node, old_generation))) => {
-                // Hey, the cached parent is our parent! We can reuse the bloom filter.
                 if old_node == layout_node_to_unsafe_layout_node(&parent) &&
                     old_generation == layout_context.shared.generation {
+                    // Hey, the cached parent is our parent! We can reuse the bloom filter.
                     debug!("[{}] Parent matches (={}). Reusing bloom filter.", tid(), old_node.0);
-                    bloom_filter.clone()
                 } else {
                     // Oh no. the cached parent is stale. I guess we need a new one. Reuse the existing
                     // allocation to avoid malloc churn.
-                    *bloom_filter = BloomFilter::new();
+                    bloom_filter.clear();
                     insert_ancestors_into_bloom_filter(&mut bloom_filter, parent, layout_context);
-                    bloom_filter
                 }
+                bloom_filter
             },
         }
     })