[memprof] Reorder MemProf sections in profile (#93640)

This patch teaches the V3 format to serialize Frames, call stacks, and
IndexedMemProfRecords, in that order.

I'm planning to use linear IDs for Frames.  That is, Frames will be
numbered 0, 1, 2, and so on in the order we serialize them.  In turn,
we will seialize the call stacks in terms of those linear IDs.

Likewise, I'm planning to use linear IDs for call stacks and then
serialize IndexedMemProfRecords in terms of those linear IDs for call
stacks.

With the new order, we can successively free data structures as we
serialize them.  That is, once we serialize Frames, we can free the
Frames' data proper and just retain mappings from FrameIds to linear
IDs.  A similar story applies to call stacks.