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 ab0a3bec1c3aba8c655ad7855fa565686a405bcf..81e14ae54822e78765a6f79cc0f35c4781a2f7fa 100644
--- a/net/base/file_stream.cc
+++ b/net/base/file_stream.cc
@@ -41,16 +41,14 @@ int FileStream::Close(const CompletionCallback& callback) {
 }
 
 bool FileStream::IsOpen() const {
-  return context_->file().IsValid();
+  return context_->IsOpen();
 }
 
-int FileStream::Seek(base::File::Whence whence,
-                     int64_t offset,
-                     const Int64CompletionCallback& callback) {
+int FileStream::Seek(int64_t offset, const Int64CompletionCallback& callback) {
   if (!IsOpen())
     return ERR_UNEXPECTED;
 
-  context_->Seek(whence, offset, callback);
+  context_->Seek(offset, callback);
   return ERR_IO_PENDING;
 }
 
@@ -88,8 +86,4 @@ int FileStream::Flush(const CompletionCallback& callback) {
   return ERR_IO_PENDING;
 }
 
-const base::File& FileStream::GetFileForTesting() const {
-  return context_->file();
-}
-
 }  // namespace net