[Remote Wipe] Add DEVICE_REMOTE_POWERWASH remote device command type

This CL just adds the enum for the command to the command proto.

DD for the feature go/remote-powerwash-command

Bug: 827523
Change-Id: Iaaa3e30ca0212039120bc36b5763ad6fb789ee37
Reviewed-on: https://chromium-review.googlesource.com/1246790Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Ivan Šandrk <isandrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595404}

components/policy/proto/device_management_backend.proto

@@ -1393,6 +1393,9 @@ message RemoteCommand {
 
     // Start Chrome Remote Desktop session (limited to Kiosk sessions only).
     DEVICE_START_CRD_SESSION = 6;
+
+    // Wipe the device (perform a powerwash).
+    DEVICE_REMOTE_POWERWASH = 7;
   }
 
   // The command type.