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Add missing enum value for background tracing state. Enum changed in:
https://chromium-review.googlesource.com/c/chromium/src/+/1659607

Change-Id: Ife1a821a9e1e9f0ac631970c768dba42c3e15c52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714329
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680237}