Move net-export thread-hopping code into NetLogFileWriter and add IO polled data.

components/net_log/BUILD.gn

 # Copyright 2015 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.
 
 static_library("net_log") {
   sources = [
     "chrome_net_log.cc",
     "chrome_net_log.h",
     "net_export_ui_constants.cc",
     "net_export_ui_constants.h",
@@ skipping 10 matching lines @@
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "net_log_file_writer_unittest.cc",
   ]
   deps = [
     ":net_log",
     "//base",
+    "//base/test:test_support",
     "//net",
+    "//net:test_support",
     "//testing/gtest",
   ]
 }