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As part of this I realized SetLockedToRoot and
SetDescendantsStayInSameRootWindow meant the same thing, so I nuked
the later.

In order to test this I need multiple display support, which isn't
quite working yet (in particular shutdown is currently
problematic).

BUG=615552
TEST=none
R=jamescook@chromium.org

Review-Url: https://codereview.chromium.org/2344783002
Cr-Commit-Position: refs/heads/master@{#419010}