The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e20eaff5a1370e290a8ac4923dfeefb40303b45e

commit e20eaff5a1370e290a8ac4923dfeefb40303b45e
Author: Catherine Chung <catherinechung@google.com>
Date: Wed Jul 19 21:32:04 2017

DeleteInstanceForTesting method for the DesktopSessionDurationTracker

This method deletes the instance and resets it to nullptr to avoid
memory leaks in tests.

Bug:  743049 
Change-Id: Ib14253155c2282dd8c4cedcc9bc25f7389ae7a28
Reviewed-on: https://chromium-review.googlesource.com/571504
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Catherine Chung <catherinechung@google.com>
Cr-Commit-Position: refs/heads/master@{#487994}
[modify] https://crrev.com/e20eaff5a1370e290a8ac4923dfeefb40303b45e/chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker.cc
[modify] https://crrev.com/e20eaff5a1370e290a8ac4923dfeefb40303b45e/chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker.h
[modify] https://crrev.com/e20eaff5a1370e290a8ac4923dfeefb40303b45e/chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker_unittest.cc

https://chromium.googlesource.com/chromium/src/+/e20eaff5a1370e290a8ac4923dfeefb40303b45e