Don't remove ref-count on download IO TaskRunner

There is no need to remove the IO TaskRunner reference.
The global io thread task_runner is never deleted.
If io TaskRunner is set, just ignore future calls.

BUG=842245

Change-Id: Ifefad6442d8efde238379218e9f11a9f3d99a75a
Reviewed-on: https://chromium-review.googlesource.com/1134197Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574402}

components/download/internal/common/download_task_runner.cc

@@ -45,20 +45,13 @@ scoped_refptr<base::SequencedTaskRunner> GetDownloadTaskRunner() {
 
 void SetIOTaskRunner(
     const scoped_refptr<base::SingleThreadTaskRunner>& task_runner) {
+  DCHECK(task_runner);
+
   base::AutoLock auto_lock(GetIOTaskRunnerLock());
-  static int count = 0;
-  if (task_runner) {
-    DCHECK(!g_io_task_runner.Get() ||
-           task_runner.get() == g_io_task_runner.Get().get());
-    count++;
-    g_io_task_runner.Get() = task_runner;
+  if (g_io_task_runner.Get())
     return;
-  }
 
-  count--;
-  DCHECK_GE(count, 0);
-  if (count == 0)
-    g_io_task_runner.Get() = nullptr;
+  g_io_task_runner.Get() = task_runner;
 }
 
 scoped_refptr<base::SingleThreadTaskRunner> GetIOTaskRunner() {

content/browser/download/download_manager_impl.cc

@@ -312,7 +312,6 @@ DownloadManagerImpl::DownloadManagerImpl(BrowserContext* browser_context)
 
 DownloadManagerImpl::~DownloadManagerImpl() {
   DCHECK(!shutdown_needed_);
-  download::SetIOTaskRunner(nullptr);
 }
 
 download::DownloadItemImpl* DownloadManagerImpl::CreateActiveItem(