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This is a reland of e40282f6.

Includes a fix to BackgroundTracingActiveScenario, which could end up enabling
TraceLog without installing the TraceEventDataSource overrides on Android, see
patch set 1 -> 2. This likely caused the crashes observed in crbug.com/997582.

Original change's description:
> tracing: Enable startup tracing for spawned child processes
>
> Enables startup tracing for child processes spawned by the browser while
> a tracing session is active.
>
> This allows us to collect early trace events in the period of time
> before the child connects to the tracing service.
>
> Bug: 968424
> Change-Id: I1708cabae77cdd2ab3a33929c28cfab04eb723e3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1755746
> Commit-Queue: Eric Seckler <eseckler@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Reviewed-by: ssid <ssid@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#689599}

Bug: 968424
Change-Id: Id69f0821671caf05238896d4a0e1b581b5d5be44
TBR: avi@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782812
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694981}