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/profile/Cargo.toml

@@ -18,6 +18,7 @@ log = { workspace = true }
 profile_traits = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
+servo_allocator = { path = "../allocator" }
 servo_config = { path = "../config" }
 time = { workspace = true }
 

components/profile/mem.rs

@@ -112,6 +112,7 @@ impl Profiler {
                     })
                     .collect();
                 let _ = sender.send(MemoryReportResult { results });
+                servo_allocator::dump_unmeasured();
                 true
             },
             ProfilerMsg::Exit => false,