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This reverts commit ded13f7d.

Reason for revert: Build Failure (Tree Closer)

Original change's description:
> Followup cleanup for ProfileSyncServiceUsesTaskScheduler experiment code.
> 
> 1) Remove sync_thread_ from the ProfileSyncService interface to avoid
> accidental use.
> 2) Enable experiment in tests to provide coverage.
> 3) Add some ScopedAllowBaseSyncPrimitives in code that needed it.
> 
> Bug: 1014464
> Change-Id: I79393edac048a2960b4f2bc7593be326f5d27419
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895797
> Commit-Queue: Oliver Li <olivierli@chromium.org>
> Reviewed-by: François Doray <fdoray@chromium.org>
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Auto-Submit: Oliver Li <olivierli@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#713459}

TBR=fdoray@chromium.org,mastiz@chromium.org,olivierli@chromium.org

Change-Id: I40c10c06af767261b7e8173db77d8a5b6224e297
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1014464
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903833Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713465}