Add a new PagedSpace called OffThreadSpace. This space will be used for
off-thread allocation -- it never marks or sweeps, and always expands
into fresh pages. Once allocation completes, this space's pages can be
merged into the old space.

The space is similar to the CompactionSpace, and merging for both is
identical, so we intrduce a new LocalSpace base class that both extend.
They differ in interaction with the sweeper and in how new pages are
acquired.

This patch adds the new space and uses it in a few unittests. Future
work will use it in the main source code.

Bug: chromium:1011762
Change-Id: Ia008cc95c6e1ef1d1e7ae305fa80fbfc1ff4be2d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1873690
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64509}