metrics: Fixes a comment for |TimerReporter::ReportMilliseconds|.

It turns out that |TimerReporter::ReportMilliseconds| does report a value
for a timer that hasn't been started (in shill,
|Metrics::NotifyDevice*Finished| calls |Stop| immediately before
|ReportMilliseconds| and it works just fine.  This is perfectly OK, it's
just that the comment didn't reflect this.  This CL fixes that.

BUG=None.
TEST=None.

Change-Id: I578d554106acdb92957bfe0197622a1054cfa9c9
Reviewed-on: https://gerrit.chromium.org/gerrit/46337


Reviewed-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Wade Guthrie <wdg@chromium.org>
Tested-by: Wade Guthrie <wdg@chromium.org>
Commit-Queue: Wade Guthrie <wdg@chromium.org>