Auto merge of #18474 - nnethercote:malloc_size_of-tweaks, r=jdm

malloc_size_of tweaks

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A couple of tweaks.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #18473 (github issue number if applicable).

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components/style/stylesheets/style_rule.rs

@@ -55,7 +55,7 @@ impl StyleRule {
             // It's safe to measure this ThinArc directly because it's the
             // "primary" reference. (The secondary references are on the
             // Stylist.)
-            n += ops.malloc_size_of(selector.thin_arc_heap_ptr());
+            n += unsafe { ops.malloc_size_of(selector.thin_arc_heap_ptr()) };
         }
 
         n += self.block.unconditional_shallow_size_of(ops) +