Refactors out the allocation and space merging parts of OffThreadFactory
into a new OffThreadHeap class. This allows a separation of concerns
between allocating/merging and initializing, and future-proofs the
factory code against off-thread allocation implementation changes (e.g.
LocalHeap).

Bug: chromium:1011762
Change-Id: I876906dbfd50f8aafe56af2e63e5fe35e4f7f8e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2157369
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67270}