Mac: Fix flaky NTP browser test where the expected NTP url changes depending...

Mac: Fix flaky NTP browser test where the expected NTP url changes depending on network connectivity

If chrome://newtab/ maps to a remote url, it fails to load in a browser_test environment and instead redirects to the local NTP, which changes the url associated with WebContents.

The NTP check in the app controller should be replaced once crbug.com/624410 is fixed.

Bug: 817328
Change-Id: If17b93d3cafa194cfa5f433d3a422108a5181394
Reviewed-on: https://chromium-review.googlesource.com/954418


Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542185}