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This paves the way to allow Variations initialization to occur without
MetricsService initialization. Deferring MetricsService inititalization
is needed for the Browser Task Scheduler work as the underlying
metrics providers will post asynchronous tasks.

BUG=636518

Review-Url: https://codereview.chromium.org/2333283005
Cr-Commit-Position: refs/heads/master@{#418866}