This moves page post-processinng into NotifyOldGenerationExpansion
and introduces a new CanExpandOldGenerationBackground.

Bug: v8:10536
Change-Id: I54761226487434955f8a0dadf7c4dbb31c7955e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2204283Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67853}