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This patch switches the background tracing subsystem to control tracing
through the Perfetto client API. The main benefit of this is being able
to specify priorities for tracing sessions (incl. legacy JSON ones),
which avoids interfering with interactive tracing sessions such as ones
from the DevTools profiler.

Bug: 1058632
Change-Id: I9c35e079a75a51fd4472011eee23fd431a720bec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404830Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Matthew Denton <mpdenton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812959}