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Previously, we collected PPS UMAs across all users. We created a throttler and used it in a 'simulated' mode.

Now that real PPS users exist, collect UMAs only on actual PPS users. This is accomplished by only creating a throttler when PPS is enabled.

BUG=470731

Review URL: https://codereview.chromium.org/1024493004

Cr-Commit-Position: refs/heads/master@{#322462}