Now that we have page-local remembered sets (due to refilling in page
granularity) we can perform all updates on the sets during compaction in
parallel without caching slots locally.

BUG=chromium:524425
LOG=N
NOTRY=true

Review URL: https://codereview.chromium.org/1811573002

Cr-Commit-Position: refs/heads/master@{#34952}