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This change adds the CpuUsageDataObvserver to SystemDataProvider.
A subsequent change will add throttling data to the CpuUsageData object

JS-side implementation of observer: https://source.chromium.org/chromium/chromium/src/+/master:chromeos/components/diagnostics_ui/resources/cpu_card.js;l=58-69

Bug: 1128204
Change-Id: I7ef493691dea63460753c56f066f6744591dbea4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518265Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826471}