This is a follow up from  issue 771966 .

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/2e4b4db0ab164c12c44f31aa3027c516fc26b993

commit 2e4b4db0ab164c12c44f31aa3027c516fc26b993
Author: Michael Lippautz <mlippautz@chromium.org>
Date: Mon Dec 04 11:39:31 2017

[heap] Untangle iterability from regular sweeping

Separates restoring iterability from sweeping a page. Since the set
of pages where iterability needs to be restored is small, it is
possible to wait and block for the task to finish if necessary.

A follow up change can now remove the fragile logic for delaying
unmapping of pages since it is guaranteed that no background task
keeps a reference to a page for restoring iterability.

Bug:  chromium:791043 
Change-Id: Ifba45594cc586df3c99e1bbb20a13b44c18dd9a1
Reviewed-on: https://chromium-review.googlesource.com/796419
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49826}
[modify] https://crrev.com/2e4b4db0ab164c12c44f31aa3027c516fc26b993/src/heap/heap.cc
[modify] https://crrev.com/2e4b4db0ab164c12c44f31aa3027c516fc26b993/src/heap/mark-compact.cc
[modify] https://crrev.com/2e4b4db0ab164c12c44f31aa3027c516fc26b993/src/heap/spaces-inl.h
[modify] https://crrev.com/2e4b4db0ab164c12c44f31aa3027c516fc26b993/src/heap/spaces.cc
[modify] https://crrev.com/2e4b4db0ab164c12c44f31aa3027c516fc26b993/src/heap/spaces.h
[modify] https://crrev.com/2e4b4db0ab164c12c44f31aa3027c516fc26b993/src/heap/sweeper.cc
[modify] https://crrev.com/2e4b4db0ab164c12c44f31aa3027c516fc26b993/src/heap/sweeper.h

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/e5a1993bffe943f79fbc3a79d5c73376baecc2c9

commit e5a1993bffe943f79fbc3a79d5c73376baecc2c9
Author: Michael Lippautz <mlippautz@chromium.org>
Date: Tue Dec 05 09:45:55 2017

[heap] Remove delayed chunks in Unmapper

The dependency between restoring iterability and the Scavenger is
explicit. Delayed chunks are thus not needed anymore.

Bug:  chromium:791043 
Change-Id: I9f2c95c1856f53299af2737f922a3cb4cc578aa5
Reviewed-on: https://chromium-review.googlesource.com/805816
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49855}
[modify] https://crrev.com/e5a1993bffe943f79fbc3a79d5c73376baecc2c9/src/heap/gc-tracer.cc
[modify] https://crrev.com/e5a1993bffe943f79fbc3a79d5c73376baecc2c9/src/heap/heap.cc
[modify] https://crrev.com/e5a1993bffe943f79fbc3a79d5c73376baecc2c9/src/heap/mark-compact.cc
[modify] https://crrev.com/e5a1993bffe943f79fbc3a79d5c73376baecc2c9/src/heap/spaces.cc
[modify] https://crrev.com/e5a1993bffe943f79fbc3a79d5c73376baecc2c9/src/heap/spaces.h

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/4d95c4b0c97eb689254a2a74c0cf1dd1acaba7f9

commit 4d95c4b0c97eb689254a2a74c0cf1dd1acaba7f9
Author: Michael Lippautz <mlippautz@chromium.org>
Date: Tue Dec 05 16:56:35 2017

[heap] Lower limit for maximum number of unmapper tasks

Bug:  chromium:791043 
Change-Id: Ica59510f9ee611d62bc65775977df7c7f99fb8ca
Reviewed-on: https://chromium-review.googlesource.com/807929
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49876}
[modify] https://crrev.com/4d95c4b0c97eb689254a2a74c0cf1dd1acaba7f9/src/heap/spaces.h