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

net/base/file_stream.cc

Index: net/base/file_stream.cc
diff --git a/net/base/file_stream.cc b/net/base/file_stream.cc
index a97d50b7886376bf6d37974d12d97c66c03db204..73d8449f7082562de7c7090ca0f719e13f18a45a 100644
--- a/net/base/file_stream.cc
+++ b/net/base/file_stream.cc
@@ -44,13 +44,11 @@ bool FileStream::IsOpen() const {
   return context_->file().IsValid();
 }
 
-int FileStream::Seek(base::File::Whence whence,
-                     int64 offset,
-                     const Int64CompletionCallback& callback) {
+int FileStream::Seek(int64 offset, const Int64CompletionCallback& callback) {
   if (!IsOpen())
     return ERR_UNEXPECTED;
 
-  context_->Seek(whence, offset, callback);
+  context_->Seek(offset, callback);
   return ERR_IO_PENDING;
 }