Move FileEnumerator to its own file, do some refactoring.

content/test/image_decoder_test.cc

Index: content/test/image_decoder_test.cc
diff --git a/content/test/image_decoder_test.cc b/content/test/image_decoder_test.cc
index 517a59ccacfbc90daa0daaec101b6fe7610640ec..a8c799c23b0ec97fccb522f84ac57cbdee188d01 100644
--- a/content/test/image_decoder_test.cc
+++ b/content/test/image_decoder_test.cc
@@ -5,6 +5,7 @@
 #include "content/test/image_decoder_test.h"
 
 #include "base/file_util.h"
+#include "base/files/file_enumerator.h"
 #include "base/files/file_path.h"
 #include "base/md5.h"
 #include "base/memory/scoped_ptr.h"
@@ -121,9 +122,9 @@ void ImageDecoderTest::SetUp() {
 std::vector<base::FilePath> ImageDecoderTest::GetImageFiles() const {
   std::string pattern = "*." + format_;
 
-  file_util::FileEnumerator enumerator(data_dir_,
-                                       false,
-                                       file_util::FileEnumerator::FILES);
+  base::FileEnumerator enumerator(data_dir_,
+                                  false,
+                                  base::FileEnumerator::FILES);
 
   std::vector<base::FilePath> image_files;
   base::FilePath next_file_name;