Revert of Enable redirection of SequencedWorkerPool to TaskScheduler in...

Revert of Enable redirection of SequencedWorkerPool to TaskScheduler in TestBrowserThreadBundle. (patchset #3 id:40001 of https://codereview.chromium.org/2769693002/ )

Reason for revert:
Caused ServiceWorkerResourceStorageDiskTest.* tests in content_unittests to crash.

See https://bugs.chromium.org/p/chromium/issues/detail?id=727994

Reverting locally fixed the crashes, so I'm going to revert this CL.

Original issue's description:
> Enable redirection of SequencedWorkerPool to TaskScheduler in TestBrowserThreadBundle.
>
> Motivation:
> - Redirection of SequencedWorkerPool to TaskScheduler is enabled in
>   browser tests and on Canary and Dev. It makes sense to enable it
>   in unit tests too.
> - It is not possible to wait until there are no more blocking pool
>   *and* TaskScheduler tasks (new tasks in the blocking pool during
>   a FlushForTesting() call on TaskScheduler and vice versa). If
>   the blocking pool is redirected to TaskScheduler, calling
>   FlushForTesting() on TaskScheduler guarantees that there are no
>   more blocking pool *and* TaskScheduler tasks.
>
> BUG=667892
>
> Review-Url: https://codereview.chromium.org/2769693002
> Cr-Commit-Position: refs/heads/master@{#475144}
> Committed: https://chromium.googlesource.com/chromium/src/+/e1aa28915a28da266d406202a07112d44d6cc018

TBR=gab@chromium.org,robliao@chromium.org,sky@chromium.org,fdoray@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=667892,727994

Review-Url: https://codereview.chromium.org/2909393003
Cr-Commit-Position: refs/heads/master@{#475816}

content/public/test/test_browser_thread_bundle.cc

@@ -10,7 +10,6 @@
 #include "base/run_loop.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/test/scoped_async_task_scheduler.h"
-#include "base/threading/sequenced_worker_pool.h"
 #include "content/browser/browser_thread_impl.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/test/test_browser_thread.h"
@@ -79,12 +78,6 @@ TestBrowserThreadBundle::~TestBrowserThreadBundle() {
   // for DestructionObservers hooked to |message_loop_| to be able to invoke
   // BrowserThread::CurrentlyOn() -- ref. ~TestBrowserThread().
   message_loop_.reset();
-
-  // Disable redirection of SequencedWorkerPools to TaskScheduler. This is
-  // required in order to reset global state so that tests following this one in
-  // this process can still manage their own SequencedWorkerPool without using
-  // TestBrowserThreadBundle.
-  base::SequencedWorkerPool::EnableForProcess();
 }
 
 void TestBrowserThreadBundle::Init() {
@@ -130,9 +123,6 @@ void TestBrowserThreadBundle::CreateThreads() {
         base::MakeUnique<base::test::ScopedAsyncTaskScheduler>();
   }
 
-  // Enable redirection of SequencedWorkerPools to TaskScheduler.
-  base::SequencedWorkerPool::EnableWithRedirectionToTaskSchedulerForProcess();
-
   if (options_ & REAL_DB_THREAD) {
     db_thread_ = base::MakeUnique<TestBrowserThread>(BrowserThread::DB);
     db_thread_->Start();