system_api: add anomaly event service interface

This also adds new event types to the metrics event protobuf,
and adds generation of C++ sources for use by the anomaly
collector.

BUG=chromium:729335
TEST=compiles, links, and runs with anomaly_collector

Change-Id: I330d9103752c9db58c6db824f69e25d5cc6bd180
Reviewed-on: https://chromium-review.googlesource.com/1239736


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>