Add generic "event of interest" enumerated histogram.

This change aims to simplify the addition of events we may
want to track in the field, more specifically hard-to-reproduce
bugs.  Before this change, this requires creating a new histogram
and registering it via histograms.xml, which is not in the Chrome OS
repositories.  With this change, a new event just requires claiming
an event name (such as ModemManagerCommandSendFailure) in
metrics_library.cc, and running "metrics_client -v <event name>"
or calling SendCrosEventToUMA(event_name).

I can make up a bug for this.  Or not.

BUG=none
TEST=compiled

Change-Id: I9c56b58310f0d22e77624edee7fe6149abd60a49
Reviewed-on: https://gerrit.chromium.org/gerrit/45322


Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>