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These tests fail flakily on linux_chromium_chromeos_rel_ng and prevents
patches landing in Chromium CQ.
Disable these tests on CrOS to allow CQ to continue.

Flakiness dashboard:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=browser_tests&tests=DownloadNotificationTest

TBR=yoshiki@chromium.org
BUG=560329

Review URL: https://codereview.chromium.org/1467183004

Cr-Commit-Position: refs/heads/master@{#361267}