GitLab

This CL:
- Refactors HelpAppTestHelper::Waiter to a general dialog window waiter.
- Renames HelpAppTestHelper to ScopedHelpAppForTest.
- Adds a test fixture that sets up a kiosk session for error screen
  tests.
- Adds a test that verifies that clicking on the "Manage certificates"
  button in the error screen opens a certificate manager window.

Bug: 959340
Change-Id: I92a9121872ff28d9c1752c222343cfc643f18501
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612599
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661111}