To let the user do special handling on the main thread e.g. Scavenging
uses different tracing categories for background/foreground threads.

Change-Id: I6c9187fd6201b5b81cd83727727fda49fcf7ff68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2405797Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69978}