Trybots are showing green when tests are failing, which could be letting cls land even if they would break the waterfall. I don't know if this is only on the Catapult CQ, or the Chromium on as well.

CL in question:
https://codereview.chromium.org/2052353003/

The 'Telemetry Tests with Stable Browser' step is shown as green, but when looking at the stdio for it, you can see that multiple tests are failing. One example:

[179/1059] telemetry.internal.backends.chrome.desktop_browser_finder_unittest.LinuxFindTest.testFindAllSystem failed unexpectedly 0.0034s:
  Chrome build location for linux_x86_64 not found. Browser will be run without Flash.
  Traceback (most recent call last):
    File "/b/build/slave/catapult/build/catapult/telemetry/telemetry/internal/backends/chrome/desktop_browser_finder_unittest.py", line 244, in testFindAllSystem
  AssertionError: Items in the second set but not the first:
  'system'
  'stable'

Taken from stdio: https://build.chromium.org/p/tryserver.client.catapult/builders/Catapult%20Linux%20Tryserver/builds/3876/steps/Telemetry%20Tests%20with%20Stable%20Browser/logs/stdio

Note that the line about the browser being run without flash shouldn't be an issue.