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If the URL of an alerting page can be unwrapped, do so. This
improves the ability of the user to tell what page is showing
the alert.

BUG=696454
TEST=the dialog in that bug is labeled.

Change-Id: I71358be49418ab6d4a88e04f317241e134e361ce
Reviewed-on: https://chromium-review.googlesource.com/797677
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520787}