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/values/generics/position.rs

@@ -6,6 +6,7 @@
 //! [`position`](https://drafts.csswg.org/css-backgrounds-3/#position)
 
 /// A generic type for representing a CSS [position](https://drafts.csswg.org/css-values/#position).
+#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug)]
 #[derive(PartialEq, ToAnimatedZero, ToComputedValue)]