Add SimpleCache index file heuristics

net/disk_cache/simple/simple_index_file.cc

Index: net/disk_cache/simple/simple_index_file.cc
diff --git a/net/disk_cache/simple/simple_index_file.cc b/net/disk_cache/simple/simple_index_file.cc
index a2913202276b22ef724a04abb6ffd0cdc34aa419..47fd3c1bdfe318c44d40f762e7c489c81c9a4e50 100644
--- a/net/disk_cache/simple/simple_index_file.cc
+++ b/net/disk_cache/simple/simple_index_file.cc
@@ -161,4 +161,21 @@ void SimpleIndexFile::WriteToDisk(const base::FilePath& index_filename,
   DCHECK(result);
 }
 
+// static
+bool SimpleIndexFile::IsIndexFileStale(const base::FilePath& index_filename) {
+  base::PlatformFileInfo dir_info;
+  base::PlatformFileInfo index_info;
+  if (!file_util::GetFileInfo(index_filename.DirName(), &dir_info))
+    return false;
+  DCHECK(dir_info.is_directory);
+  if (!file_util::GetFileInfo(index_filename, &index_info))
+    return false;
+
+  // Index file last_modified must be equal to the directory last_modified since
+  // the last operation we do is ReplaceFile in the
+  // SimpleIndexFile::WriteToDisk().
+  // If not true, we need to restore the index.
+  return index_info.last_modified >= dir_info.last_modified;
+}
+
 }  // namespace disk_cache