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This allows for the Browser Task Scheduler to setup and redirect after
SetupFieldTrials to get variations data but before SetupMetrics, which
posts tasks to the blocking pool.

This harmonizes iOS with
https://codereview.chromium.org/2342993002/

BUG=636518

Review-Url: https://codereview.chromium.org/2376123004
Cr-Commit-Position: refs/heads/master@{#422890}