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/properties/longhand/counters.mako.rs

@@ -37,6 +37,7 @@
         use values::specified::Attr;
 
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
         #[derive(Clone, Debug, Eq, PartialEq, ToComputedValue)]
         pub enum ContentItem {
             /// Literal string content.
@@ -97,6 +98,7 @@
             }
         }
 
+        #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         #[derive(Clone, Debug, Eq, PartialEq, ToComputedValue)]
         pub enum T {
@@ -236,6 +238,7 @@
     use style_traits::ToCss;
     use values::CustomIdent;
 
+    #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
     #[derive(Clone, Debug, PartialEq)]
     pub struct SpecifiedValue(pub Vec<(CustomIdent, specified::Integer)>);
 
@@ -245,6 +248,7 @@
         use values::CustomIdent;
 
         #[derive(Clone, Debug, PartialEq)]
+        #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct T(pub Vec<(CustomIdent, i32)>);