Blimp: make the Engine terminate on disconnect.
Blimp: make the Engine terminate on disconnect.

The engine can't be reused across connections until a session recovery protocol is in place. This CL makes the engine quit on disconnect, so that later clients will have a clean initialized state.

BUG=576342
R=maniscalco@chromium.org,wez@chromium.org

Review URL: https://codereview.chromium.org/1581563002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#380340}

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