Reland: mus: Enable system modal windows

When a modal window does not have a transient parent, it will be modal
to the whole system.

BUG=548402

Committed: https://crrev.com/aad86d4a28e819ff7b1d6cef106c7d88da2d48aa
Cr-Commit-Position: refs/heads/master@{#390963}

Review-Url: https://codereview.chromium.org/1818333002
Cr-Commit-Position: refs/heads/master@{#392007}
17 files changed