Added a timer-based report to the metrics library.

Timer encapsulates a timer with basic functionality.
TimerReporter subclasses it for also sending UMA reports.

BUG=chromium-os:18800
TEST=Unit test 'timer_test' has been included

Change-Id: I9de9a2a7388721ba1476fe706a8d12788d2176ad
Reviewed-on: http://gerrit.chromium.org/gerrit/5161


Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Bruno Pontes Soares Rocha <bpontes@google.com>
Reviewed-by: Bruno Pontes Soares Rocha <bpontes@google.com>