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Add a new test, it makes use of chrome.app.window.create() with a
sandboxed page. It checks no window is returned to the background app.

There were no tests trying to open a new window with a sandboxed
document already.

Bug: 820995
Change-Id: I0e59723ee23b31845545774b3ff3dfede0278af0
Reviewed-on: https://chromium-review.googlesource.com/982627
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547657}