The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d69292fa5ab883d726fca5aebf76a42412c2348d

commit d69292fa5ab883d726fca5aebf76a42412c2348d
Author: Catherine Chung <catherinechung@google.com>
Date: Thu Jun 29 00:19:20 2017

Added observer to DesktopSessionDurationTracker.

Currently, the DesktopSessionDurationTracker only keeps track of session
time for a browsing instance, not an overall Chrome session. Adding
an observer for the class will allow the tracker to tell observers
when a session ends.

The first client for this observer is the NewTabFeatureEngagementTracker,
which uses the signals to determine when new users have accumulated
2 hours of usage in order to show them in-product assistance. The
observer is tested in its unittests.
https://chromium-review.googlesource.com/c/549057

Bug:  736438 
Change-Id: Id51b48e160c25f80e9557d94ce1becfdb9dd2f2e
Reviewed-on: https://chromium-review.googlesource.com/546540
Commit-Queue: Catherine Chung <catherinechung@google.com>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483215}
[modify] https://crrev.com/d69292fa5ab883d726fca5aebf76a42412c2348d/chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker.cc
[modify] https://crrev.com/d69292fa5ab883d726fca5aebf76a42412c2348d/chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker.h
[modify] https://crrev.com/d69292fa5ab883d726fca5aebf76a42412c2348d/chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/72a5f214a18417091ae236aa169e62b10d247d15

commit 72a5f214a18417091ae236aa169e62b10d247d15
Author: Catherine Chung <catherinechung@google.com>
Date: Sat Jul 29 02:25:26 2017

Using a ScopedObserver in NewTabTracker

The NewTabTracker uses an DesktopSessionDurationTracker::Observer
to be notified when an active session starts or ends. It was changed to
a ScopedObserver to ensure all observers are removed from the sources
it has been added to when it is destroyed.

Bug:  736438 
Change-Id: I68618f7436dddb4a27378f090c59f1c63510abcb
Reviewed-on: https://chromium-review.googlesource.com/583902
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490587}
[modify] https://crrev.com/72a5f214a18417091ae236aa169e62b10d247d15/chrome/browser/feature_engagement/new_tab/new_tab_tracker.cc
[modify] https://crrev.com/72a5f214a18417091ae236aa169e62b10d247d15/chrome/browser/feature_engagement/new_tab/new_tab_tracker.h
[modify] https://crrev.com/72a5f214a18417091ae236aa169e62b10d247d15/chrome/browser/feature_engagement/new_tab/new_tab_tracker_unittest.cc