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After pressing ui::VKEY_BROWSER_BACK, the test needs to wait for the
navigation to finish + for the load to finish.  Both can be accomplished
with the help of content::TestNavigationObserver.

Fixed: 846623
Change-Id: I1e7ab80ad4caa37b12c2e99a50eba2393b4bc2bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511253
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822923}