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ShutdownOnIOThread may be called more than once. If this happens,
the MessageLoop observer will be null during the second call.
This ensures that we don't dereference it in that case.

BUG=None
TBR=ben@chromium.org

Review-Url: https://codereview.chromium.org/2259753002
Cr-Commit-Position: refs/heads/master@{#412768}