This flattens the json output to one result record as a dict. In
the past several records with different arch/mode combinations
could be run, but this is deprecated since several releases.

We also drop storing the arch/mode information in the record as it
isn't used on the infra side for anything.

This was prepared on the infra side by:
https://crrev.com/c/2453562

Bug: chromium:1132088
Change-Id: I944514dc00a671e7671bcdbcaa3a72407476d7ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2456987Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70402}