Simplify the organization of //mojo_file_utils.

mojo/file_utils/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.
 
 import("//mojo/public/mojo_application.gni")
 
 # file_utils is a helper library for utility functions relating to the Mojo
 # files service.
 source_set("file_utils") {
   sources = [
+    "file_util.cc",
     "file_util.h",
-    "lib/file_util.cc",
   ]
 
   public_deps = [
     "//mojo/services/files/interfaces:interfaces",
   ]
 }
 
 mojo_native_application("file_utils_apptests") {
   output_name = "file_utils_apptests"
 
   testonly = true
 
   sources = [
-    "tests/file_util_test_base.cc",
-    "tests/file_util_test_base.h",
-    "tests/file_util_unittest.cc",
+    "file_util_unittest.cc",
   ]
 
   deps = [
     ":file_utils",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/application:test_support_standalone",
     "//mojo/public/cpp/utility",
     "//testing/gtest",
   ]
 }