We don't need to reset the entire age table, but merely the pages that
are known to contain young objects. This must improve memory use with
generational GC enabled.

The CL is a prerequisite for another CL that'll increase the size of
the age-table.

Bug: chromium:1029379
Change-Id: Ibb5b607af20380c3936b7396b3d9767f6f17c44b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695268
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81106}