Enable user change background image in settings page in Aura build.

chrome/browser/resources/options2/browser_options.html

 <div id="settings" class="page" hidden>
   <header>
     <h1 i18n-content="settingsTitle"></h1>
     <span id="browser-options-search-field-container"
         class="search-field-container">
       <input id="search-field" type="search"
           i18n-values="placeholder:searchPlaceholder;
                        aria-label:searchPlaceholder" incremental>
     </span>
   </header>
@@ skipping 80 matching lines @@
                 i18n-content="startupSetPages">
             </button>
           </span>
         </label>
       </div>
     </div>
   </section>
 </if>
   <section>
     <h3 i18n-content="sectionTitleAppearance"></h3>
+    <div>
+<if expr="pp_ifdef('use_ash')">
+      <button id="set-wallpaper" i18n-content="setWallpaper"></button>
+</if>
 <if expr="not pp_ifdef('toolkit_views') and is_posix and not is_macosx">
-    <div>
       <button id="themes-gallery" i18n-content="themesGallery"></button>
       <button id="themes-GTK-button"
           i18n-content="themesGTKButton"></button>
       <button id="themes-reset"
           i18n-content="themesSetClassic"></button>
-    </div>
 </if>
 <if expr="pp_ifdef('toolkit_views') or is_win or is_macosx">
-    <div>
       <button id="themes-gallery" i18n-content="themesGallery"></button>
       <button id="themes-reset" i18n-content="themesReset"></button>
+</if>
     </div>
-</if>
     <div class="settings-row">
       <span i18n-content="homePage"></span>
       <select id="home-page-select">
         <option value="none" i18n-content="homePageNone"></option>
         <option value="ntp" i18n-content="homePageNtp"></option>
         <option value="url"></option>
         <option value="choose" i18n-content="homePageChoose"></option>
       </select>
     </div>
     <div class="checkbox"><label>
@@ skipping 523 matching lines @@
   </section>
 </if>
 </div>  <!-- advanced-settings-container -->
 </div>  <!-- advanced-settings -->
 <footer>
   <button id="advanced-settings-expander" class="link-button"
       i18n-content="showAdvancedSettings">
   </button>
 </footer>
 </div>