Move performance scenario traces into one category

Also makes the tracks for ScenarioScope::kGlobal global, instead of
nesting them under the browser process track.

R=etiennep

Bug: 365586676
Change-Id: Ib09cc5426afcb260313a831da43ceb21ecb127e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6432566
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1443780}