Sweeper cannot assume that platform never changes, so that we can
support using testing-specific platforms.
Instead, the sweeper gets the current platform from HeapBase on sweeping
start. The platform is set to nullptr whenever sweeping is not active.

Bug: chromium:1056170
Change-Id: I749e1dbfa204635fbb446a8c383aaa2548a717be
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2767139Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73486}