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This reverts commit abd57f32.

Reason for revert: Will reland similar CL once the FILE thread is also deprecated (we prefer to deprecate all browser threads at once).

Original change's description:
> Remove BrowserThread::FILE_USER_BLOCKING/CACHE/DB.
> 
> These threads are deprecated and no longer used.
> 
> NOPRESUBMIT=true
> 
> Bug: 689520
> Change-Id: I55b05b70626f91c8a71d56642c851bb786dbfcaa
> Reviewed-on: https://chromium-review.googlesource.com/660597
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Commit-Queue: Francois Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#501245}

TBR=jam@chromium.org,fdoray@chromium.org

Change-Id: I4a0605974792092af8780595fcccef5a6dda3d87
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 689520
Reviewed-on: https://chromium-review.googlesource.com/663598Reviewed-by: Francois Doray <fdoray@chromium.org>
Commit-Queue: Francois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501263}