Move FileEnumerator to its own file, do some refactoring.

chrome/browser/extensions/external_pref_loader.cc

Index: chrome/browser/extensions/external_pref_loader.cc
diff --git a/chrome/browser/extensions/external_pref_loader.cc b/chrome/browser/extensions/external_pref_loader.cc
index 32cb24cd7502760378ea53571a5602ffc015463b..46eeb0dfe6bc5a575c5c2f1cdc261acfa102241c 100644
--- a/chrome/browser/extensions/external_pref_loader.cc
+++ b/chrome/browser/extensions/external_pref_loader.cc
@@ -6,6 +6,7 @@
 
 #include "base/bind.h"
 #include "base/file_util.h"
+#include "base/files/file_enumerator.h"
 #include "base/files/file_path.h"
 #include "base/json/json_file_value_serializer.h"
 #include "base/json/json_string_value_serializer.h"
@@ -35,10 +36,10 @@ std::set<base::FilePath> GetPrefsCandidateFilesFromFolder(
     return external_extension_paths;
   }
 
-  file_util::FileEnumerator json_files(
+  base::FileEnumerator json_files(
       external_extension_search_path,
       false,  // Recursive.
-      file_util::FileEnumerator::FILES);
+      base::FileEnumerator::FILES);
 #if defined(OS_WIN)
   base::FilePath::StringType extension = UTF8ToWide(std::string(".json"));
 #elif defined(OS_POSIX)