The perf_event_attr struct has union fields:
- sample_freq/sample_period
- wakeup_watermark/wakeup_events

To determine which of these fields is used, the |freq| and |watermark| bits are used. These flags also have corresponding fields in the protobuf.

However, when converting from struct to protobuf and vice versa, the code is currently looking at the destination flags rather than the source flags.