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In production, FileDescriptorWatchers can be leaky and post
BLOCK_SHUTDOWN tasks in response to their notification.

Stopping the service thread requires some non-trivial task lifetime
fixes to do it correctly, so this was deemed the next best quick fix
for redirection.

BUG=717380

Review-Url: https://codereview.chromium.org/2857103005
Cr-Commit-Position: refs/heads/master@{#469461}