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This is a new implementation of DesktopCloudPolicyStore. It has
- Different path of policy cache
  The policy files are store as
    "$user_data_dir/Policy/Machine Level User Cloud Policy" and
     "$user_data_dir/Policy/Machine Level User Cloud Policy Signing Key".
- Global dm_token and client_id
  It will load the policy cache from disk iff the global token is existed.
  And it will validate the dm_token and client_id with the global ones after
  store/load.

Bug: 812641
Change-Id: If02ad77388fc197f20dc25c04c1f852e1827253a
Reviewed-on: https://chromium-review.googlesource.com/940442
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541582}