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

Change-Id: I001b669e318bf88f39358ddb96482ec5ed4b62a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418986Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809328}