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Sorry, I reverted wrong cl.

BUG: 751948
TBR=bcwhite@chromium.org,holte@chromium.org,olivierrobin@chromium.org

This is a reland of 433d6917
Original change's description:
> Make MetricsLog not need a PrefService.
> 
> Move system profile persistence to MetricsService.
> 
> Bug: 
> Change-Id: I5c31dc53de267dd9699a83af926f8a575bab18a0
> Reviewed-on: https://chromium-review.googlesource.com/592689
> Commit-Queue: Steven Holte <holte@chromium.org>
> Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
> Reviewed-by: Brian White <bcwhite@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#491573}

Change-Id: Iea3ebc29e92f3e12fc56f7b7ba767c531f216041
Reviewed-on: https://chromium-review.googlesource.com/598759
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491684}