The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e2ea864f13a06487314df6a196dd39a554067520

commit e2ea864f13a06487314df6a196dd39a554067520
Author: Istiaque Ahmed <lazyboy@chromium.org>
Date: Mon Jun 18 22:00:12 2018

Fix a leak in OneShotEvent class.

Once Signal()ed, OneShotEvent should drop all the references to
the tasks, otherwise OneShotEvent::tasks_ would continue to hold
references to users of OneShotEvent::Post(), for the lifetime
of OneShotEvent.

This CL clears |tasks_| once Signal() is called, dropping those
references correctly. In future, OneShotEvent::Post() will take
OnceClosure instead of (repeating) Closure.

This CL also adds a regression test for the same.

Bug:  853649 
Change-Id: I10270318ff8393a9af0b8da299a1866df936b273
Reviewed-on: https://chromium-review.googlesource.com/1103606
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568184}
[modify] https://crrev.com/e2ea864f13a06487314df6a196dd39a554067520/extensions/common/one_shot_event.cc
[modify] https://crrev.com/e2ea864f13a06487314df6a196dd39a554067520/extensions/common/one_shot_event_unittest.cc