Revert of Use TaskScheduler instead of WorkerPool in cache_util.cc.

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

Reason for revert:
suspect for net_unittest failures

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

[mikecase (-- gone --), 2017-01-11 22:11:54]

The CQ bit was checked by mikecase@chromium.org

[mikecase (-- gone --), 2017-01-11 22:11:54]

Created Revert of Use TaskScheduler instead of WorkerPool in cache_util.cc.

[commit-bot: I haz the power, 2017-01-11 22:12:51]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-11 22:29:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-01-11 22:29:04]

Failed to apply patch for net/disk_cache/backend_unittest.cc:
While running git apply --index -p1;
  error: patch failed: net/disk_cache/backend_unittest.cc:14
  error: net/disk_cache/backend_unittest.cc: patch does not apply

Patch:       net/disk_cache/backend_unittest.cc
Index: net/disk_cache/backend_unittest.cc
diff --git a/net/disk_cache/backend_unittest.cc
b/net/disk_cache/backend_unittest.cc
index
14580c0512ea5d7646882fb08514da6b99e4580c..e92dd9b2f59ce8d07ee293ce6e536c16402ec1ce
100644
--- a/net/disk_cache/backend_unittest.cc
+++ b/net/disk_cache/backend_unittest.cc
@@ -14,7 +14,6 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/test/mock_entropy_provider.h"
-#include "base/test/scoped_task_scheduler.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/thread_restrictions.h"
@@ -83,9 +82,6 @@
 // Tests that can run with different types of caches.
 class DiskCacheBackendTest : public DiskCacheTestWithCache {
  protected:
-  DiskCacheBackendTest()
-      : scoped_task_scheduler_(base::MessageLoop::current()) {}
-
   // Some utility methods:
 
   // Perform IO operations on the cache until there is pending IO.
@@ -153,9 +149,6 @@
   void BackendDisabledAPI();
 
   void BackendEviction();
-
- private:
-  base::test::ScopedTaskScheduler scoped_task_scheduler_;
 };
 
 int DiskCacheBackendTest::GeneratePendingIO(net::TestCompletionCallback* cb) {
@@ -2054,8 +2047,6 @@
 // blockfile caches.
 #if !defined(OS_ANDROID)
 TEST_F(DiskCacheTest, SimpleCacheControlJoin) {
-  base::test::ScopedTaskScheduler scoped_task_scheduler(
-      base::MessageLoop::current());
   base::Thread cache_thread("CacheThread");
   ASSERT_TRUE(cache_thread.StartWithOptions(
                   base::Thread::Options(base::MessageLoop::TYPE_IO, 0)));