This is a strict improvement over the status quo, though we'll still
eventually consume as much memory as before, since we don't try to
evict at all until we hit the max size. We'll fix that soon, but it's
worth landing this separately because:

• It's useful to separate out regressions in the growing logic from in any new complex eviction logic.
• This patch alone is a very large (~100MB) win on AWSY.

Depends on D10854