Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/download_request_core.cc

Index: content/browser/download/download_request_core.cc
diff --git a/content/browser/download/download_request_core.cc b/content/browser/download/download_request_core.cc
index ded98502547fb259e1e1580363f508003056323d..92a9491ea0cefc7d14089fd402827d4165ab8ec6 100644
--- a/content/browser/download/download_request_core.cc
+++ b/content/browser/download/download_request_core.cc
@@ -23,6 +23,7 @@
 #include "content/browser/download/download_manager_impl.h"
 #include "content/browser/download/download_request_handle.h"
 #include "content/browser/download/download_stats.h"
+#include "content/browser/download/download_task_runner.h"
 #include "content/browser/loader/resource_dispatcher_host_impl.h"
 #include "content/browser/service_manager/service_manager_context.h"
 #include "content/public/browser/browser_thread.h"
@@ -307,8 +308,7 @@ bool DownloadRequestCore::OnResponseStarted(
 
   // Create the ByteStream for sending data to the download sink.
   std::unique_ptr<ByteStreamReader> stream_reader;
-  CreateByteStream(base::ThreadTaskRunnerHandle::Get(),
-                   BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
+  CreateByteStream(base::ThreadTaskRunnerHandle::Get(), GetDownloadTaskRunner(),
                    kDownloadByteStreamSize, &stream_writer_, &stream_reader);
   stream_writer_->RegisterCallback(
       base::Bind(&DownloadRequestCore::ResumeRequest, AsWeakPtr()));