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This is called when a tab opened by way of window.open() is asked to
close (window.close()).

BUG=none
TEST=covered by test

Change-Id: I0d97069faed7a77edfef2e2932328e271d5d6db1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913601
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715086}