system_api: dbus: add metrics_event protobuffer

This is used for signaling memory pressure events from Chrome to
memd (or other interested party).

BUG=chromium:729335
TEST=compiles and runs correctly with chromium changes that use it

Change-Id: I00aee977045977d606e3f09eb8a4215c54ea5ee2
Reviewed-on: https://chromium-review.googlesource.com/1186024
Commit-Ready: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
1 file changed