In pprofextended proto, consider moving Location.type_index to Mapping

[aalexand]

We define it as

  // Type of frame (e.g. kernel, native, python, hotspot, php). Index into string table.
  uint32 type_index = 6;

I can't easily think of a case where the type of locations would be different for the given mapping. This could probably be moved to the mapping message which would also save some profile size since locations tend to come in much larger quantities compared to mappings.

Related: Could this type be an attribute? Both mapping and location have attributes attached.