update_engine: Update to build against libchrome-271506.

BUG=chromium:375032
TEST=`FEATURES=test emerge-$BOARD update_engine`

Change-Id: Ia7be302c12b439ad8aece51c9cbe128086626620
Reviewed-on: https://chromium-review.googlesource.com/201045
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/prefs_unittest.cc b/prefs_unittest.cc
index a3595a3..fa66bfd 100644
--- a/prefs_unittest.cc
+++ b/prefs_unittest.cc
@@ -29,9 +29,8 @@
   }
 
   bool SetValue(const string& key, const string& value) {
-    return file_util::WriteFile(prefs_dir_.Append(key),
-                                value.data(), value.length()) ==
-        static_cast<int>(value.length());
+    return base::WriteFile(prefs_dir_.Append(key), value.data(),
+                           value.length()) == static_cast<int>(value.length());
   }
 
   base::FilePath prefs_dir_;