Measure the UA cache.

ServoStyleSetSizes now has two uses, one for the Stylist, and one for the UA
cache, and so the patch removes 'Stylist' from the field names.

Example output from about:memory:

> +----1,359,608 B (00.55%) -- layout
> |    +----756,488 B (00.31%) -- style-sheet-cache [2]
> |    +----393,968 B (00.16%) -- servo-ua-cache
> |    |    +--234,496 B (00.10%) -- element-and-pseudos-maps
> |    |    +---59,648 B (00.02%) -- revalidation-selectors
> |    |    +---58,320 B (00.02%) -- invalidation-map
> |    |    +---30,752 B (00.01%) -- other
> |    |    +---10,752 B (00.00%) -- precomputed-pseudos

components/style/selector_parser.rs

@@ -112,6 +112,7 @@ pub trait ElementExt: Element<Impl=SelectorImpl> + Debug {
 }
 
 /// A per-functional-pseudo map, from a given pseudo to a `T`.
+#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct PerPseudoElementMap<T> {
     entries: [Option<T>; SIMPLE_PSEUDO_COUNT],