metrics_daemon: report cumulative daily use.

This had been lost in the previous changes.  Also, the meaning
of DailyUse changes: it is now cumulative daily use.  But we should
revise these stats so I think it's OK for now.

BUG=chromium:339588
TEST=unit tests

Change-Id: I4894fef0ffb237633abcf949c01dd7ea5f5e5d6e
Reviewed-on: https://chromium-review.googlesource.com/195005


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