Move ManagedBookmarksTracker from policy/core/browser to (new) bookmarks/managed.

After https://codereview.chromium.org/769153007, it handles more than policy.

BUG=443606

Review URL: https://codereview.chromium.org/961683002

Cr-Commit-Position: refs/heads/master@{#321539}
15 files changed