Remove the file thread dependency from FileNetLogObserver.

components/net_log/net_export_file_writer_unittest.cc

Index: components/net_log/net_export_file_writer_unittest.cc
diff --git a/components/net_log/net_export_file_writer_unittest.cc b/components/net_log/net_export_file_writer_unittest.cc
index 17f3b4b2fddf90a108584f40cc9b581d0e6bc055..cf61b07034491b3addb2ac9f0e3c8f1a4e7d1509 100644
--- a/components/net_log/net_export_file_writer_unittest.cc
+++ b/components/net_log/net_export_file_writer_unittest.cc
@@ -16,8 +16,7 @@
 #include "base/files/scoped_temp_dir.h"
 #include "base/json/json_reader.h"
 #include "base/json/json_string_value_serializer.h"
-#include "base/message_loop/message_loop.h"
-#include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/values.h"
@@ -437,8 +436,7 @@ class NetExportFileWriterTest : public ::testing::Test {
   TestStateObserver test_state_observer_;
 
  private:
-  // Allows tasks to be posted to the main thread.
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 };
 
 TEST_F(NetExportFileWriterTest, InitFail) {