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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.

The corresponding iOS change will be in a later CL

BUG=636518

Review-Url: https://codereview.chromium.org/2342993002
Cr-Commit-Position: refs/heads/master@{#419511}