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Always place it in the center of the "root window for new windows", which
is the display where the user last interacted with a window.

This fixes a regression where the dialog could be centered over a tabbed
browser window. It also fixes a couple cases where adding a new Wi-Fi
network from the status tray menu on a secondary monitor would open the
dialog on the primary monitor.

Added some basic test coverage to NetworkConfigView.

Bug: 740316
Test: chrome browser_tests. Also manually add a new Wi-Fi network during OOBE, at the login screen, and after login on both primary and secondary displays.
Change-Id: I60f2070dea9f39da586c97ff8b178fc34c5fb325
Reviewed-on: https://chromium-review.googlesource.com/569025
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486507}