Extensions: Migrate off DEPRECATED_DISABLE_UNKNOWN_FROM_SYNC

This disable reason hasn't been in active use since M48, but it
turns out our previous migration code didn't catch all cases.
This CL adds (hopefully) more robust migration code.

Bug: 860198
Change-Id: I3b81a8cc3941459d501cc3b4d8381250b374f0ab
Reviewed-on: https://chromium-review.googlesource.com/1124686
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573164}
2 files changed