Restore lost metric change from used KiB to Percent.
Restore lost metric change from used KiB to Percent.

Somehow the change to the metric calculation didn't
get included with the change that renamed the metric.

BUG=

Review URL: https://codereview.chromium.org/1700093002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#375667}

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