Expect local_ntp_test_utils::GetFinalNtpUrl to avoid flaky test.

Expecting a chrome://newtab/ url to check for a new tab page could cause
tests to be flaky because the NTP URL could be chrome://newtab/ or
chrome-search://local-ntp/local-ntp.html. This change makes the
expectation use the GetFinalNtpUrl util instead to ensure the correct
URL is expected.

Bug: 1078301
Change-Id: I9cc31a7399a7b79cd832327c67f59959ef97e5eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391601
Commit-Queue: Lu Huang <luhua@microsoft.com>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806762}