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This change wires the ManagedBookmarksTracker to the ChromeBookmarkClient.
The tracker keeps the new managed_node in sync with the ManagedBookmarks
policy, and the ChromeBookmarkClient publishes that node as an extra node
for the BookmarkModel.

Also cleaned up some duplicated constants used by the managed bookmarks code.

BUG=49598

Review URL: https://codereview.chromium.org/319543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275382 0039d316-1c4b-4281-b951-d872f2087c98