Fix extra_memory on string

Summary: It wasn't originally clear if the extra_memory at the end of a string entry should go under extra_memory or not. I think the answer is not, since that seems to work out better. Leave it as an explicit comment returning 0 since it's a corner case either way.

Reviewed By: krallin

Differential Revision: D30900300

fbshipit-source-id: 8bd01b2ad7d9b3371ec797eed794c6c49afd3b45

Author: ndmitchell

starlark/src/values/types/string.rs

@@ -294,7 +294,10 @@ impl<'v> StarlarkValue<'v> for StarlarkStr {
     }
 
     fn extra_memory(&self) -> usize {
-        self.len
+        // We don't include the extra_memory for the size because it is
+        // allocated inline in the Starlark heap (which knows about it),
+        // not on the malloc heap.
+        0
     }
 
     fn length(&self) -> anyhow::Result<i32> {