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The ExecuteScriptAndExtractBool calls in HasMinimalUiButtons tests
were sometimes failing.

We now wait for navigation, and close app windows when they
are no longer required.

Bug: 1120504
Change-Id: Ifd869e6ec9e480edd9e85fbd9116b237e38e70e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532480
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826505}