Exposing a new easyUnlock.setupConnectionGetDeviceAddress JS API.

This CL exposes a new JS API to find the bluetooth address for the
remote device corresponding to a given connection id.

This is necesssary as the bluetooth address of a device changes after
it's paired (but it's updated in the underlying
BluetoothLowEnergyConnection object).

BUG=528277

Review URL: https://codereview.chromium.org/1370133005

Cr-Commit-Position: refs/heads/master@{#351549}
diff --git a/components/proximity_auth/connection.h b/components/proximity_auth/connection.h
index a79ce188..532e03d9 100644
--- a/components/proximity_auth/connection.h
+++ b/components/proximity_auth/connection.h
@@ -54,6 +54,9 @@
   // Disconnects from the remote device.
   virtual void Disconnect() = 0;
 
+  // The bluetooth address of the connected device.
+  virtual std::string GetDeviceAddress();
+
   Status status() const { return status_; }
 
  protected: