deps: cherry-pick 98c40a4bae915 from V8 upstream

Original commit message:

    [platform] Return task runners as shared_ptr

    At the moment, task runners are returned as unique_ptr. This is
    inconvenient, however. In all implementations I did, the platform holds
    a shared pointer of the task runner and wraps it in a wrapper class just
    to return it as a unique_ptr. With this CL the platform API is changed
    to return a shared_ptr directly.

    R=rmcilroy@chromium.org

    Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
    Change-Id: Ide278db855199ea239ad0ae14d97fd17349dac8c
    Reviewed-on: https://chromium-review.googlesource.com/768867


    Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#49366}

Refs: https://github.com/v8/v8/commit/98c40a4bae915a9762c73de3307300c61e4fd91a

PR-URL: https://github.com/nodejs/node/pull/17134
Fixes: https://github.com/nodejs/node-v8/issues/24


Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>