Auto merge of #18495 - nnethercote:derive-PropertyDeclaration, r=jdm

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.

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- [ ] These changes fix #__ (github issue number if applicable).

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components/style/values/computed/mod.rs

@@ -383,6 +383,7 @@ impl From<GreaterThanOrEqualToOneNumber> for CSSFloat {
 }
 
 #[allow(missing_docs)]
+#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[derive(Clone, ComputeSquaredDistance, Copy, Debug, PartialEq, ToCss)]
 pub enum NumberOrPercentage {