Re-enable EnterpriseEnrollmentTest.TestProperPageGetsLoadedOnEnrollmentFailure

Use WaitForStep, this should avoid functional flakiness.

Note that ChromeOS MSAN timeouts were increased recently
(https://chromium-review.googlesource.com/c/1289083).

Bug: 690634
Change-Id: Id96718281b0473d8bb34d41d6985ccdeec82cb20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541177
Commit-Queue: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#647716}