Auto merge of #18932 - nnethercote:rename-size_of_is_0, r=emilio

Rename `size_of_is_0!` as `malloc_size_of_is_0!`.

The new name makes it clearer that it comes from the `malloc_size_of`
crate.

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

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components/style/gecko_string_cache/mod.rs

@@ -389,4 +389,4 @@ impl From<*mut nsAtom> for Atom {
     }
 }
 
-size_of_is_0!(Atom);
+malloc_size_of_is_0!(Atom);

components/style/properties/longhand/color.mako.rs

@@ -72,7 +72,7 @@ pub mod system_colors {
 
     // It's hard to implement MallocSizeOf for LookAndFeel_ColorID because it
     // is a bindgen type. So we implement it on the typedef instead.
-    size_of_is_0!(SystemColor);
+    malloc_size_of_is_0!(SystemColor);
 
     impl ToCss for SystemColor {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {