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

net/base/mock_file_stream.h

Index: net/base/mock_file_stream.h
diff --git a/net/base/mock_file_stream.h b/net/base/mock_file_stream.h
index 39ea1f3eb3dc4e78c6d1b56e05869f4979f5fe5b..4572b866a27ee964e730e88865dcf8d872864180 100644
--- a/net/base/mock_file_stream.h
+++ b/net/base/mock_file_stream.h
@@ -28,9 +28,7 @@ class MockFileStream : public FileStream {
   ~MockFileStream() override;
 
   // FileStream methods.
-  int Seek(base::File::Whence whence,
-           int64 offset,
-           const Int64CompletionCallback& callback) override;
+  int Seek(int64 offset, const Int64CompletionCallback& callback) override;
   int Read(IOBuffer* buf,
            int buf_len,
            const CompletionCallback& callback) override;