[tracing] Support background tracing after incognito close

This CL makes a few changes to support background tracing again
once incognito is closed.
- TracingDelegate::IsRecordingAllowed considers the tracing start time
- Emit triggers on incognito start and stop
- Stop rules still apply when a nested scenario is active: This allows stopping when incognito starts; this changes the semantic of nested scenario, but there isn't any use case that rely on the existing behavior.

Change-Id: I0d4d0b4b9ed2a58e1e725450d08f2f2b7126b6ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6645661
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Bo Liu <boliu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1475312}