An example build is: https://luci-milo.appspot.com/swarming/task/376cf93d4b40d110

This build succeeded on BuildBot: https://uberchromegw.corp.google.com/i/tryserver.chromium.win/builders/win_chromium_rel_ng/builds/491314

An excerpt from the failure is:
ChromeUserLevel: no_pv -> install_chrome_user -> chrome_user_installed_not_inuse -> uninstall_chrome_user -> clean
 ... [0717/220906:test_installer.py(144)] Verifying state no_pv
[0717/220906:test_installer.py(110)] Beginning action install_chrome_user
[0717/220912:test_installer.py(113)] Finished action install_chrome_user
[0717/220912:test_installer.py(144)] Verifying state chrome_user_installed_not_inuse
FAIL
[0717/220913.205:ERROR:uninstall.cc(133)] Failed to delete path: C:\Users\chrome-bot\AppData\Local\Chromium\Application\61.0.3160.0\Installer\setup.exe
Traceback (most recent call last):
  File "uninstall_chrome.py", line 68, in <module>
    sys.exit(main())
  File "uninstall_chrome.py", line 63, in main
    'status %d.' % exit_status)
Exception: Could not uninstall Chrome. The installer exited with status 20.
[0717/220913:test_installer.py(233)] Error cleaning up an old install with:
Traceback (most recent call last):
  File "e:\b\s\w\ir\cache\builder\win\src\chrome\test\mini_installer\test_installer.py", line 229, in RunCleanCommand
    RunCommand(command, variable_expander)
  File "e:\b\s\w\ir\cache\builder\win\src\chrome\test\mini_installer\test_installer.py", line 169, in RunCommand
    expanded_command, exit_status))
Exception: Command python uninstall_chrome.py --chrome-long-name="Chromium" --no-error-if-absent   returned non-zero exit status 1

My read on this is that Chrome successfully uninstalled, but that the uninstaller failed to clean up the installer's "setup.exe" afterwards. There aren't really any logs about the reason for this particular failure. The build environment here should be similar to our Windows trybots.

I'm not familiar with the installer, so I'm going to add the Installer component for help figuring out what's wrong here.