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This reverts commit d9339152.

Reason for revert: <INSERT REASONING HERE>
Causing compile error here:
https://build.chromium.org/p/chromium.chrome/builders/Google%20Chrome%20Win/builds/19349

Original change's description:
> Replace GetBlockingPool and FILE thread with post_task API
> 
> BUG=689520,667892
> 
> Change-Id: I1143fbc8c5f7c12dfdd8e5764d58c9b372d3cb16
> Reviewed-on: https://chromium-review.googlesource.com/544782
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Reviewed-by: Greg Thompson <grt@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#481889}

TBR=xiyuan@chromium.org,grt@chromium.org

Change-Id: I4fbdb87edffff078ca60bb16c10628b5b13da00a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 689520, 667892
Reviewed-on: https://chromium-review.googlesource.com/546115Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481902}