Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//gpu/vulkan/features.gni")
 import("//media/media_options.gni")
 import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
@@ skipping 623 matching lines @@
     "download/download_net_log_parameters.cc",
     "download/download_net_log_parameters.h",
     "download/download_request_core.cc",
     "download/download_request_core.h",
     "download/download_request_handle.cc",
     "download/download_request_handle.h",
     "download/download_resource_handler.cc",
     "download/download_resource_handler.h",
     "download/download_stats.cc",
     "download/download_stats.h",
+    "download/download_task_runner.cc",
+    "download/download_task_runner.h",
     "download/download_worker.cc",
     "download/download_worker.h",
     "download/drag_download_file.cc",
     "download/drag_download_file.h",
     "download/drag_download_util.cc",
     "download/drag_download_util.h",
     "download/mhtml_extra_parts_impl.cc",
     "download/mhtml_extra_parts_impl.h",
     "download/mhtml_generation_manager.cc",
     "download/mhtml_generation_manager.h",
@@ skipping 1493 matching lines @@
   if (!is_component_build) {
     public_deps = [
       ":browser",
     ]
   } else {
     public_deps = [
       "//third_party/leveldatabase",
     ]
   }
 }