Move FileEnumerator to its own file, do some refactoring.

base/files/file_enumerator.cc

+// Copyright (c) 2013 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.
+
+#include "base/files/file_enumerator.h"
+
+#include "base/file_util.h"
+
+namespace base {
+
+FileEnumerator::FileInfo::~FileInfo() {
+}
+
+bool FileEnumerator::ShouldSkip(const FilePath& path) {
+  FilePath::StringType basename = path.BaseName().value();
+  return basename == FILE_PATH_LITERAL(".") ||
+         (basename == FILE_PATH_LITERAL("..") &&
+          !(INCLUDE_DOT_DOT & file_type_));
+}
+
+}  // namespace base