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This CL modifies the OpenPopup (aka window.open helper) to avoid using
unchecked WaitForLoadStop(), replacing it with waiting on a
TestNavigationObserver for the specific URL being opened.

Bug: 1014186
Change-Id: Ib28f9fe251336d1e1f3a0e6326792881e13fd671
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2165578
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Auto-Submit: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762845}