Update definition of tracked preference support.
Update definition of tracked preference support.

BUG=507660 (ref. discussion @ https://codereview.chromium.org/1227973003/#msg20)

Review URL: https://codereview.chromium.org/1253393003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#340474}

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