[tracing] Add Tracing.Background.Scenario.Trigger histogram

To record individual trigger in addition to scenario events.
Drive-by: update BackgroundTracingScenarioNameHash values

Bug: 1418116
Change-Id: I4e1a737dc88e09e7dc42a608d3a4af86cedb24b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5426594


Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283171}