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This also renames kThirdPartyVPN -> kExtension and kArcVPN -> kArc

This is in preparation for merging vpn_list.mojom into
cros_network_config and using it in Ash and Settings.

Bug: 979314
Change-Id: I2a74156080d59a4ae2f757c8410e01036dedcd68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775319Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691790}