GUID often have content in the higher bits of a 64-bit entry so using
the unabbrev encoding is inefficient (lots of VBR control bits).
Instead, use an abbrev with two 32-bit fixed width chunks.
The abbrev also helps encode the "count" in one place instead of
in every record.
Reduces size of distributed backend summary files by 8.7% in one
example app.
Co-authored-by: Jan Voung <jvoung@google.com>
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