cashew: add metrics support

This CL adds a MetricsManager class wrapping libmetrics and adds an
initial metric, Cros.Network.Cashew.UsageRequestStatus, that tracks the
success or failure of usage API requests.  We'll add additional metrics
in subsequent CLs.

BUG=chromium-os:9887
TEST=ran unit tests and performed manual testing on device, looking at
cashew logs and chrome://histograms.

Change-Id: I3c5b2fdb23d4203ba8466a831896630a9c5bab7e
Reviewed-on: http://gerrit.chromium.org/gerrit/4459
Tested-by: Vince Laviano <vlaviano@chromium.org>
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>