Only support seeking file streams from the beginning of the file.

storage/browser/fileapi/local_file_stream_writer.cc

Index: storage/browser/fileapi/local_file_stream_writer.cc
diff --git a/storage/browser/fileapi/local_file_stream_writer.cc b/storage/browser/fileapi/local_file_stream_writer.cc
index 71c6bfbbb04c8f41ce9f587f2588747cdd2d0723..72eb6cd1de46d04a9f5dee155c0d58e412136db3 100644
--- a/storage/browser/fileapi/local_file_stream_writer.cc
+++ b/storage/browser/fileapi/local_file_stream_writer.cc
@@ -152,11 +152,10 @@ void LocalFileStreamWriter::InitiateSeek(
     return;
   }
 
-  int result = stream_impl_->Seek(base::File::FROM_BEGIN, initial_offset_,
-                                  base::Bind(&LocalFileStreamWriter::DidSeek,
-                                             weak_factory_.GetWeakPtr(),
-                                             error_callback,
-                                             main_operation));
+  int result = stream_impl_->Seek(
+      initial_offset_,
+      base::Bind(&LocalFileStreamWriter::DidSeek, weak_factory_.GetWeakPtr(),
+                 error_callback, main_operation));
   if (result != net::ERR_IO_PENDING) {
     has_pending_operation_ = false;
     error_callback.Run(result);