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Revert of Use TaskScheduler instead of WorkerPool in cache_util.cc. (patchset #3 id:40001 of https://codereview.chromium.org/2610503002/ )

Reason for revert:
It looks like this CL may be related to a unit test failure on this build:
https://build.chromium.org/p/chromium.win/builders/Win10%20Tests%20x64/builds/7492
- Sheriff

Original issue's description:
> Use TaskScheduler instead of WorkerPool in cache_util.cc.
>
> The following traits are used:
>
> Priority: BACKGROUND
>   User won't notice if this task takes an arbitrarily long time
>   to complete.
>
> Shutdown behavior: CONTINUE_ON_SHUTDOWN
>   Tasks posted with this mode which have not started executing before
>   shutdown is initiated will never run. Tasks with this mode running at
>   shutdown will be ignored (the worker will not be joined).
>
>   Note: Tasks that were previously posted to base::WorkerPool should
>   use this shutdown behavior because this is how base::WorkerPool
>   handles all its tasks.
>
> May Block:
>   Tasks posted with MayBlock() may block. This includes but is not
>   limited to tasks that wait on synchronous file I/O operations:
>   read or write a file from disk, interact with a pipe or a socket,
>   rename or delete a file, enumerate files in a directory, etc. This
>   trait isn't required for the mere use of locks.
>
> BUG=659191
>
> Review-Url: https://codereview.chromium.org/2610503002
> Cr-Commit-Position: refs/heads/master@{#442985}
> Committed: https://chromium.googlesource.com/chromium/src/+/8d1a5a1e352ee6b2aee40b42d9fdc4b6e0c97fed

TBR=gavinp@chromium.org,fdoray@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=659191

Review-Url: https://codereview.chromium.org/2626163002
Cr-Commit-Position: refs/heads/master@{#443009}