Add a device policy for system-wide trace collection.

This changes adds a new policy, DeviceSystemWideTracingEnabled,
controlling whether it's allowed to start a system-wide trace
collection with the the Perfetto system tracing service (traced)
from within the browser.

Bug: b:147789115
Test: browser_tests: DeviceSystemWideTracingEnabledPolicyTest.*

Change-Id: I64ee8c8d1fe1ba415124a361986e8d8ee987d5cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612787


Commit-Queue: Chinglin Yu <chinglinyu@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#850954}