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 * Use signal-safe logging.
 * Ignore signals that are not to the main thread.

As far as I can tell, the runloop's quit closure is safe to run
in a signal handler, so that should be OK.

BUG= internal b/26253582

Review URL: https://codereview.chromium.org/1542233002

Cr-Commit-Position: refs/heads/master@{#367089}