battery: Reference the spec in navigator_battery.idl

TBR=timvolodine@chromium.org

Change-Id: I36020efa88606fa42a29c451f3e66164290ba8a7
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1106139
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#568429}

third_party/blink/renderer/modules/battery/navigator_battery.idl

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// https://w3c.github.io/battery/#the-navigator-interface
 [
     ImplementedAs=NavigatorBattery
 ] partial interface Navigator {