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This CL does the following:
- Allows adding "VPN" to ONC's DisableNetworkTypes list.
- Adds a VPN device state entry in the list returned by
  CrosNetworkConfig::GetDeviceStateList() to be used by UI.
- If VPN is disabled by policy:
  * Removes VPN pod from the system tray.
  * Removes VPN entry from the network summary page in settings.
  * Disables (but not removes) the "Add OpenVPN/L2TP..." button
    in the network summary page.

Bug: 1064893, 1082327
Change-Id: I0441e3ab0eae27b2af7eafc7dd7b0f2ad5fcd5b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2190216
Commit-Queue: Omar Morsi <omorsi@google.com>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781806}