Add a fieldtrial testing entry for SensitiveContentRedaction.

This CL was created on behalf of ntfschr.
Please review the CL and approve it if it looks good.
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Change-Id: I62b17b053598d2fffe5548ca390ec9953fed56e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8594877
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Auto-Submit: Chrome Metrics Config Manager <chrome-metrics-config-manager-server@google.com>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1721262}
NOKEYCHECK=True
GitOrigin-RevId: 94a2938e9c6f7e5168ab227849571885dfc7146e