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This is a cleanup change in preparation for [ http://crbug.com/450509 ], where
metrics will be logged in non-ChromeOS-only code.

BUG=450509
TEST=compiles
R=tengs@chromium.org

Review URL: https://codereview.chromium.org/887093004

Cr-Commit-Position: refs/heads/master@{#314759}