allocator: Add optional heap allocation measurement tracking. (#38727)

Add an off-by-default allocator mode that tracks all live allocations
with sizes and associated stack traces. We also track if each allocation
is visited as part of a measuring heap usage in `about:memory`, allowing
us to report on allocations that are not tracked yet. Right now the list
of untracked allocations is dumped to stdout; I have a python script
coming in a separate PR which makes it easier to perform analysis on the
massive output.

Testing: Manually tested with `./mach build -d --features
servo_allocator/allocation-tracking` and visiting about:memory.
Part of: #11559

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/shared/profile/mem.rs

@@ -293,7 +293,7 @@ pub fn perform_memory_report<F: FnOnce(&mut MallocSizeOfOps)>(f: F) {
     let seen_pointer = move |ptr| SEEN_POINTERS.with(|pointers| !pointers.borrow_mut().insert(ptr));
     let mut ops = MallocSizeOfOps::new(
         servo_allocator::usable_size,
-        None,
+        servo_allocator::enclosing_size,
         Some(Box::new(seen_pointer)),
     );
     f(&mut ops);