[CrOS Tether] Add HasConnectedToHost property for Tether networks.

components/onc/docs/onc_spec.md

 # Open Network Configuration
 
 [TOC]
 
 ## Objective
 
 We would like to create a simple, open, but complete format to describe
 multiple network configurations for WiFi, Ethernet, Cellular,
 Bluetooth/WiFi-Direct, and VPN connections in a single file format, in order
 to simplify and automate network configuration for users.
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 * **BatteryPercentage**
     * (optional, read-only) - **integer**
     * The battery percentage of the device providing the tether hotspot in the
       range [0, 100].
 
 * **Carrier**
     * (optional, read-only) - **string**
     * The name of the cellular carrier when the hotspot is provided by a
       cellular connection.
 
+* **HasConnectedToHost**
+    * (read-only) - **boolean**
+    * If *true*, the current device has already connected to a Tether network
+      created by the same external device which is providing this Tether
+      network.
+
 * **SignalStrength**
     * (optional, read-only) - **integer**
     * The current signal strength for the hotspot's connection in the range
       [0, 100]. Note that this value refers to the strength of the signal
       between the external device and its data provider, not the strength of the
       signal between the current device and the external device.
 
 
 ## Bluetooth / WiFi Direct Networks
 
@@ skipping 337 matching lines @@
 is transmitted or saved to disk should be secure. On client device, when
 user names for connections that are user-specific are persisted to disk,
 they should be stored in a location that is encrypted. Users can also opt in
 these cases to not save their user credentials in the config file and will
 instead be prompted when they are needed.
 
 ## Authors
 
 * pneubeck@chromium.org
 * stevenjb@chromium.org