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No functional change intended, just duplicate code reduction.
Adds the common signal handler to broker_process.cc, since all
it does is call back into BrokerProcess methods.

Change-Id: Ie8f1604888465fb9996c34a7f0f42811b8cddb30
Reviewed-on: https://chromium-review.googlesource.com/773109
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517454}