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Previously, we read a variation param from the "BrowserScheduler" study
to determine whether all tasks should have USER_BLOCKING priority.
With this CL, we instead use a base::Feature.

Benefits:
- Study that controls the feature doesn't have to be "BrowserScheduler".
- Enabling/disabling a feature is less verbose than setting a variation
  params with the new GCL config format.

Bug: 902441
Change-Id: Ib38dcfdf097c1ca92784a22c3501b644b077f744
Reviewed-on: https://chromium-review.googlesource.com/c/1321032
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606988}