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This requires fixing a test that tried to drag between different root
windows while the default capture client was installed, which can't
handle that.  Looking through history, it seems like the whole reason
AuraTestHelper _didn't_ set a root window here originally was because
this test existed, and it was worked around instead of fixed.

TBH, I can't quite remember why I needed to do this at this point,
but it seems more correct regardless.

Bug: none
Change-Id: Ibc14703786d48f3283ec393434781ed02eae5ddb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2117068
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752831}