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Chrome also detects when the connection to mus is lost and further
chrome needs to control when the shell it shutdown, so adds the option
to set a callback that controls what should happen when connection to
mus is lost.

I think there is more that needs to happen, but this is a good start.

BUG=709593
TEST=covered by tests
R=jonross@chromium.org

Review-Url: https://codereview.chromium.org/2846893004
Cr-Commit-Position: refs/heads/master@{#468013}