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The task runner (`base::sequence_manager::internal::TaskQueueImpl::TaskRunner`
 -- child of `base::SingleThreadTaskRunner`) being held in
`unfreezable_message_filter_` refers to an object of
`base::sequence_manager::internal::TaskQueueImpl::GuardedTaskPoster`,
which holds an object of `base::internal::OperationsController`, which
then holds an object of `base::WaitableEvent`, which finnally holds an
open mach port.

The issue also impacts other systems like Windows / Linux.

Bug: 1063577
Change-Id: I5e34c71953835d9312360d349072ec4809f00d11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132536Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755872}