Update settings-dropdown-menu to observe array mutations of menuOptions

Currently changes to menuOptions via array mutations do not cause
shouldDisableMenu_() to be called again. In particular, this causes
and issue with the resolution dropdown for CrOS Display Settings since
we rebuild the list of resolutions by assigning an empty array and then
calling the 'push' array mutation.

Fixed: 1105149
Change-Id: I9e275454254af56ea3aab5f83176a51cd447d0c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327790
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Auto-Submit: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793154}

chrome/browser/resources/settings/controls/settings_dropdown_menu.html

@@ -39,7 +39,7 @@
     </template>
     <select class="md-select" id="dropdownMenu" on-change="onChange_"
         aria-label$="[[label]]"
-        disabled="[[shouldDisableMenu_(disabled, menuOptions, pref.*)]]">
+        disabled="[[shouldDisableMenu_(disabled, menuOptions.*, pref.*)]]">
       <template is="dom-repeat" items="[[menuOptions]]">
         <option value="[[item.value]]">[[item.name]]</option>
       </template>