[A11y] Add a11y description for disabled mobile-data toggle

This CL fixes issue with mobile data toggle screen-reader
callouts not being descriptive about how the toggle can be
enabled. Fixed this by adding a description about enabling
bluetooth.

Screenshot: https://screenshot.googleplex.com/w8Sw5qbeq9C3oy7.png

Fixed: 1111853
Change-Id: I64b32b14e20dda610f7f2d33a197289140181eba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2482082
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819904}

chrome/browser/resources/settings/chromeos/internet_page/network_summary_item.html

@@ -85,6 +85,7 @@
         <cr-toggle id="deviceEnabledButton"
             class="margin-matches-padding"
             aria-label$="[[getToggleA11yString_(deviceState)]]"
+            aria-describedby$="[[getToggleA11yDescribedBy_(deviceState)]]"
             checked="[[deviceIsEnabled_(deviceState)]]"
             disabled="[[!enableToggleIsEnabled_(deviceState)]]"
             on-change="onDeviceEnabledChange_">

chrome/browser/resources/settings/chromeos/internet_page/network_summary_item.js

@@ -290,6 +290,22 @@ Polymer({
     return '';
   },
 
+  /**
+   * @param {!OncMojo.DeviceStateProperties|undefined} deviceState
+   * @return {string}
+   * @private
+   */
+  getToggleA11yDescribedBy_(deviceState) {
+    // Use network state text to describe toggle for uninitialized tether
+    // device. This announces details about enabling bluetooth.
+    if (this.enableToggleIsVisible_(deviceState) &&
+        deviceState.type === mojom.NetworkType.kTether &&
+        deviceState.deviceState == mojom.DeviceStateType.kUninitialized) {
+      return 'networkState';
+    }
+    return '';
+  },
+
   /**
    * @return {boolean} True if VPNs are disabled by policy and the current
    *     device is VPN.