Add FileReader interface and implement FileSystemFileReader

webkit/blob/blob_url_request_job.cc

Index: webkit/blob/blob_url_request_job.cc
diff --git a/webkit/blob/blob_url_request_job.cc b/webkit/blob/blob_url_request_job.cc
index 49f9b04c19876e8c13671a038f732e06ca89ddba..65f8e0855d944c57366be5491f962560e90d080b 100644
--- a/webkit/blob/blob_url_request_job.cc
+++ b/webkit/blob/blob_url_request_job.cc
@@ -239,7 +239,7 @@ void BlobURLRequestJob::DidGetFileItemLength(size_t index, int64 result) {
   // in the real time.
   int64 item_length = static_cast<int64>(item.length);
   if (item_length == -1)
-    item_length = result;
+    item_length = result - item.offset;
 
   // Cache the size and add it to the total size.
   DCHECK_LT(index, item_length_list_.size());