Move FileEnumerator to its own file, do some refactoring.

third_party/leveldatabase/env_chromium.cc

Index: third_party/leveldatabase/env_chromium.cc
diff --git a/third_party/leveldatabase/env_chromium.cc b/third_party/leveldatabase/env_chromium.cc
index d88f46027b7ecc7944d1469376090650e5e58b61..9a351c66ac829b55c4820265b6ea9ecf330a4cd8 100644
--- a/third_party/leveldatabase/env_chromium.cc
+++ b/third_party/leveldatabase/env_chromium.cc
@@ -10,6 +10,7 @@
 #include "base/at_exit.h"
 #include "base/debug/trace_event.h"
 #include "base/file_util.h"
+#include "base/files/file_enumerator.h"
 #include "base/files/file_path.h"
 #include "base/lazy_instance.h"
 #include "base/memory/ref_counted.h"
@@ -590,8 +591,8 @@ bool ChromiumEnv::FileExists(const std::string& fname) {
 Status ChromiumEnv::GetChildren(const std::string& dir,
                                 std::vector<std::string>* result) {
   result->clear();
-  ::file_util::FileEnumerator iter(
-      CreateFilePath(dir), false, ::file_util::FileEnumerator::FILES);
+  base::FileEnumerator iter(
+      CreateFilePath(dir), false, base::FileEnumerator::FILES);
   base::FilePath current = iter.Next();
   while (!current.empty()) {
     result->push_back(FilePathToString(current.BaseName()));