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

net/base/mock_file_stream.cc

Index: net/base/mock_file_stream.cc
diff --git a/net/base/mock_file_stream.cc b/net/base/mock_file_stream.cc
index 19ff24e1982740bd607927d6d25ccb16756b3c7a..c699b463d593f169bc4f450958d83cfafd3a8625 100644
--- a/net/base/mock_file_stream.cc
+++ b/net/base/mock_file_stream.cc
@@ -35,14 +35,13 @@ MockFileStream::MockFileStream(
 MockFileStream::~MockFileStream() {
 }
 
-int MockFileStream::Seek(base::File::Whence whence,
-                         int64_t offset,
+int MockFileStream::Seek(int64_t offset,
                          const Int64CompletionCallback& callback) {
   Int64CompletionCallback wrapped_callback =
       base::Bind(&MockFileStream::DoCallback64,
                  weak_factory_.GetWeakPtr(), callback);
   if (forced_error_ == OK)
-    return FileStream::Seek(whence, offset, wrapped_callback);
+    return FileStream::Seek(offset, wrapped_callback);
   return ErrorCallback64(wrapped_callback);
 }