Measure PropertyDeclaration more thoroughly.

This patch replaces the handwritten MallocSizeOf implementation for
PropertyDeclaration with a derived one, which gives much more thorough
measurement.

This requires (a) deriving MallocSizeOf for a *lot* of additional types (most
of which already have `derive(HeapSizeOf)` in Servo builds), and (b)
implementing MallocSizeOf for a few more types in the `malloc_size_of` crate.

These changes would significantly improve the reporting coverage for gmail if
it weren't for the fact that SpecifiedUrl isn't measured due to a lack of
clarity about its fields; that can be fixed as a follow-up once bug 1397971 has
landed.

components/style_traits/cursor.rs

@@ -17,6 +17,7 @@ macro_rules! define_cursor {
     ) => {
         /// https://drafts.csswg.org/css-ui/#cursor
         #[derive(Clone, Copy, Debug, Eq, PartialEq)]
+        #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
         #[cfg_attr(feature = "servo", derive(Deserialize, Serialize, HeapSizeOf))]
         #[repr(u8)]
         #[allow(missing_docs)]