Move WriteFile and WriteFileDescriptor from file_util to base namespace.

ash/desktop_background/desktop_background_controller_unittest.cc

Index: ash/desktop_background/desktop_background_controller_unittest.cc
diff --git a/ash/desktop_background/desktop_background_controller_unittest.cc b/ash/desktop_background/desktop_background_controller_unittest.cc
index 3c41f3d74bc3f16a92852b7a61e632a1f28f3f3f..1c4810598756e008649fb3165bedc6eac567b903 100644
--- a/ash/desktop_background/desktop_background_controller_unittest.cc
+++ b/ash/desktop_background/desktop_background_controller_unittest.cc
@@ -215,7 +215,7 @@ class DesktopBackgroundControllerTest : public test::AshTestBase {
       return false;
     }
 
-    size_t bytes_written = file_util::WriteFile(
+    size_t bytes_written = base::WriteFile(
         path, reinterpret_cast<const char*>(&output[0]), output.size());
     if (bytes_written != output.size()) {
       LOG(ERROR) << "Wrote " << bytes_written << " byte(s) instead of "