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

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}

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 @@ std::unique_ptr<disk_cache::BackendImpl> CreateExistingEntryCache(
 // 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 @@ class DiskCacheBackendTest : public DiskCacheTestWithCache {
   void BackendDisabledAPI();
 
   void BackendEviction();
-
- private:
-  base::test::ScopedTaskScheduler scoped_task_scheduler_;
 };
 
 int DiskCacheBackendTest::GeneratePendingIO(net::TestCompletionCallback* cb) {
@@ -2054,8 +2047,6 @@ TEST_F(DiskCacheTest, WrongVersion) {
 // 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)));

net/disk_cache/cache_util.cc

@@ -12,8 +12,8 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
-#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread_restrictions.h"
+#include "base/threading/worker_pool.h"
 
 namespace {
 
@@ -136,13 +136,8 @@ bool DelayedCacheCleanup(const base::FilePath& full_path) {
     return false;
   }
 
-  base::PostTaskWithTraits(
-      FROM_HERE, base::TaskTraits()
-                     .WithShutdownBehavior(
-                         base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN)
-                     .WithPriority(base::TaskPriority::BACKGROUND)
-                     .MayBlock(),
-      base::Bind(&CleanupCallback, path, name_str));
+  base::WorkerPool::PostTask(
+      FROM_HERE, base::Bind(&CleanupCallback, path, name_str), true);
   return true;
 }