In some workloads the Unmapper could reach kMaxUnmapperTasks at which
point it wouldn't start any new tasks and not free any more memory
until the next major GC. It could lead to a large buildup of memory in
the Unmapper.

Bug: v8:7440
Change-Id: I23fda67b2e27824c04ac886d7e111bb01188be74
Reviewed-on: https://chromium-review.googlesource.com/913490Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51308}