Use the metrics library mock rather than inventing one ourselves.

BUG=None
TEST=Ran on x86-alex-paladin trybot.

Change-Id: I85aafefb369f4651331afeae9e2ec21a0df7fcbe
Reviewed-on: https://gerrit.chromium.org/gerrit/47208
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
1 file changed