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I've just noticed that crrev.com/606339 causes a crash, and that's
because PrintWindowHierarchy starts from a root window, which
do not have a parent, but it tries to call IsToplevelWindow() which
assumes the existence of the parent.

BUG=none
TEST=manually

Change-Id: Ie4460302674d057a6c7960e0b012c80b63e06a2c
Reviewed-on: https://chromium-review.googlesource.com/c/1327560Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606651}