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/counter_style/mod.rs

@@ -342,6 +342,7 @@ impl ToCss for System {
 }
 
 /// https://drafts.csswg.org/css-counter-styles/#typedef-symbol
+#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
 #[derive(Clone, Debug, Eq, PartialEq, ToComputedValue)]
 pub enum Symbol {
     /// <string>
@@ -489,6 +490,7 @@ impl Parse for Fallback {
 }
 
 /// https://drafts.csswg.org/css-counter-styles/#descdef-counter-style-symbols
+#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
 #[derive(Clone, Debug, Eq, PartialEq, ToComputedValue)]
 pub struct Symbols(pub Vec<Symbol>);