Part of this is that with USS, Sync uses *much* less memory of its own :)
It would still be nice to count it, especially for pseudo-USS data types.

Here's where we do it for directory (i.e. pre-USS) data types:
https://cs.chromium.org/chromium/src/components/sync/engine_impl/sync_manager_impl.cc?rcl=cac0db13cfc9e321cf40231653c9242731eb247a&l=990

And to provide some more context -- we regularly get reports from the field of sync-related components using a lot of memory. e.g. in https://memlog.corp.google.com/report?reportid=fd0fd79bbf07a39a we see that there are ~600,000 bookmarks using 182MB of memory. The Sync MDP allows us to verify that this memory usage is intentional and WAI and not a potential memory leak.