GSuite apps have observed an increase in memory usage reported via performance.memory with the rollout of site isolation. This is expected, but it would generally be helpful to expose a signal to pages about the number of browsing contexts in the current process together with the memory measurement. This would allow pages to better attribute what portion of memory usage is due to them directly, vs. just being accounted from unrelated similar domain contexts.

Ideally would be able to understand both top-level browsing contexts only, and total number including iframes.

This has been proposed before in the GSuite performance.memory perspective doc: https://docs.google.com/document/d/1Y7KKXVmMY5QFCLh73hleHanmAg5Icu5IgBP7q4LFUYI/edit#bookmark=id.wl8fkv21l476