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/specified/border.rs

@@ -17,6 +17,7 @@ use values::specified::{AllowQuirks, Number, NumberOrPercentage};
 use values::specified::length::{Length, LengthOrPercentage};
 
 /// A specified value for a single side of the `border-width` property.
+#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[derive(Clone, Debug, PartialEq, ToCss)]
 pub enum BorderSideWidth {