Close all shell connections when an embedded app quits
Close all shell connections when an embedded app quits

ShellConnection can't outlive its ShellClient.

BUG=609514
R=ben@chromium.org

Review-Url: https://codereview.chromium.org/1953793002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#391854}

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