Introduce background Download process to android

content/app/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/chrome_build.gni")
 
 content_app_sources = [
   "android/app_jni_registrar.cc",
   "android/app_jni_registrar.h",
   "android/child_process_service.cc",
   "android/child_process_service.h",
   "android/content_jni_onload.cc",
   "android/content_main.cc",
   "android/content_main.h",
+  "android/download_main.cc",
   "android/library_loader_hooks.cc",
   "android/library_loader_hooks.h",
   "content_main.cc",
   "content_main_runner.cc",
   "mac/mac_init.mm",
   "mac/mac_init.h",
   "mojo/mojo_init.cc",
   "mojo/mojo_init.h",
 ]
 
@@ skipping 76 matching lines @@
   source_set("child") {
     visibility = [ "//content/public/app:child" ]
 
     sources = content_app_sources
     configs += content_app_extra_configs
     deps = content_app_deps
 
     defines = [ "CHROME_MULTIPLE_DLL_CHILD" ]
   }
 }