[CrOS Cellular] Fix broken cellular settings page.

The cellular "device" properties (e.g., IMEI, ICCID) were no longer
being displayed. This regression was caused by a refactor [1] which
renamed a property but was still referencing the old property's name.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1460401

Bug: 979681
Change-Id: I619bc42d2235686fa51002dad3785d6c260fa284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1681976
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673440}

chrome/browser/resources/settings/internet_page/internet_detail_page.js

@@ -1439,7 +1439,7 @@ Polymer({
    * @private
    */
   getDeviceFields_: function() {
-    if (!this.networkProperties ||
+    if (!this.networkProperties_ ||
         this.networkProperties_.Type !== CrOnc.Type.CELLULAR) {
       return [];
     }