I'll simply disable the test on Mac, because disabling the test based on
Debug/Release difference seems uncommon.

Well, this is flaky on Mac Release, too.

https://findit-for-me.appspot.com/waterfall/check-flake?master_name=chromium.mac&builder_name=Mac10.9+Tests&build_number=28341&step_name=unit_tests+on+Mac-10.9&test_name=DesktopEngagementServiceTest.TestTimeoutDiscount

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/029a4964f2eabf97aa126e1b3becc28509425c74

commit 029a4964f2eabf97aa126e1b3becc28509425c74
Author: yutak <yutak@chromium.org>
Date: Wed Sep 14 10:24:28 2016

Disable DesktopEngagementServiceTest.TestTimeoutDiscount on Mac.

This test is flaky on Mac debug bot.

BUG= 646758 
TBR=gayane@chromium.org,asvitkine@chromium.org

Review-Url: https://codereview.chromium.org/2338213003
Cr-Commit-Position: refs/heads/master@{#418523}

[modify] https://crrev.com/029a4964f2eabf97aa126e1b3becc28509425c74/chrome/browser/metrics/desktop_engagement/desktop_engagement_service_unittest.cc

gayane@, can you take a look?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/68de0eb04f00eed4467bb743eccaec1b92dba132

commit 68de0eb04f00eed4467bb743eccaec1b92dba132
Author: gayane <gayane@chromium.org>
Date: Thu Sep 15 22:03:14 2016

Fix DesktopEngagementServiceTest.TestTimeoutDiscount unittests

Fixing the flaky unittest by correcting the wrong measurement units.
This CL also enables the test for mac bots.

BUG= 645538 ,  646758 

Review-Url: https://codereview.chromium.org/2344673002
Cr-Commit-Position: refs/heads/master@{#419000}

[modify] https://crrev.com/68de0eb04f00eed4467bb743eccaec1b92dba132/chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker_unittest.cc
[modify] https://crrev.com/68de0eb04f00eed4467bb743eccaec1b92dba132/tools/metrics/histograms/histograms.xml