[3PCD] Migrate old 3PC blocking pref on onboarding

We can't directly rely on the CookieControlsMode enum here because it
creates a circular dependency, but this enum is completely static at
this point anyway, so I've just commented it inline.

Change-Id: I6ada9fa8004c422becd38aa199eda1c0df05ec71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5366090
Commit-Queue: Fiona Macintosh <fmacintosh@google.com>
Auto-Submit: Fiona Macintosh <fmacintosh@google.com>
Reviewed-by: Kevin Graney <kmg@google.com>
Cr-Commit-Position: refs/heads/main@{#1272563}