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This adds a mechanism to report statistically-interesting
events to Chrome OS listeners.  A Chrome OS daemon (such
as the memory daemon "memd") can register as a client
of the Metrics Event Service, and receive signals on the
occurrence of related events (for instance, a tab discard).

This CL adds the supporting machinery and its first use,
signaling of a tab discard.

CQ-DEPEND=CL:1199965
BUG=chromium:729335
TEST=ran on DUT, verified that D-Bus signal is sent as expected

Change-Id: I6eddf4b0d18aa4c8a3fd4aab7f0770d87ddc7aeb
Reviewed-on: https://chromium-review.googlesource.com/1062078
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589639}