Currently this observer uses 80% of initial new space capacity as
step size. But this means that after the first minor GC this will most
likely decouple from the current new space size since the allocation
counter isn't reset after a GC and surviving objects aren't
accounted.

Use 64K as step-size since this should be large enough to not cause
regression but it should still work for Scavenger and Minor MC such
that a step invocation will be performed close to reaching 80% of
new space capacity.

Bug: v8:12612
Change-Id: I4abc17eaeded90e0f72d9467a4410159ef0e6dda
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3879618Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83079}