Do not store page_start_ in SlotSet anymore, when needed this address
can be calculated cheaply and be passed in from the caller.

Bug: v8:9454
Change-Id: I4cdb010e4126680d8df500e40ae3d0bc884cf501
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1838731Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64103}