Add setting to isolate zoom changes by default.

chrome/browser/resources/settings/site_settings/zoom_levels.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
+<link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../settings_shared_css.html">
 <link rel="import" href="site_settings_behavior.html">
 <link rel="import" href="site_settings_prefs_browser_proxy.html">
 
 <dom-module id="zoom-levels">
   <template>
     <style include="settings-shared">
       :host {
         display: block;
       }
 
+      /**
+       * When per-origin zooming is disabled, we change the opacity of any
+       * existing zoom level entries to indicate that they are not being used.
+       * In order to indicate that the buttons can still be used to clear the
+       * entries, we do not change the opacity of the buttons.
+       */
+      #listContainer[disabled] .favicon-image,
+      #listContainer[disabled] .middle,
+      #listContainer[disabled] .zoom-label {
+        opacity: var(--settings-disabled-opacity);
+      }
+
       .zoom-label {
         -webkit-margin-end: 16px;
       }
 
       .empty-message {
         margin-top: 15px;
       }
     </style>
-    <div class="list-frame vertical-list" id="listContainer">
+    <div class="settings-box first two-line">
+      <div class="start secondary">
+        [[getScopeLabel_(zoomScopeIsPerOrigin_)]]
+      </div>
+      <paper-toggle-button id="toggle" checked="{{zoomScopeIsPerOrigin_}}"
+          on-change="toggleZoomScope_" disabled$="[[isGuest_]]">
+      </paper-toggle-button>
+    </div>
+    <div class="list-frame vertical-list" id="listContainer"
+        disabled$="[[!zoomScopeIsPerOrigin_]]">
       <template is="dom-repeat" items="[[sites_]]" id="list"
           rendered-item-count="{{renderedCount}}">
         <div class="list-item">
           <div class="favicon-image"
               style$="[[computeSiteIcon(item.originForFavicon)]]">
           </div>
           <div class="middle">
             <div>[[item.displayName]]</div>
           </div>
           <div class="zoom-label">[[item.zoom]]</div>
           <div>
             <paper-icon-button icon="cr:clear"
                 on-tap="removeZoomLevel_">
           </div>
         </div>
       </template>
-      <template is="dom-if" if="{{!renderedCount}}">
+      <template is="dom-if"
+          if="{{showNoZoomedSites_(renderedCount, zoomScopeIsPerOrigin_)}}">
         <div class="empty-message" id="empty">
           $i18n{siteSettingsNoZoomedSites}
         </div>
       </template>
     </div>
   </template>
   <script src="zoom_levels.js"></script>
 </dom-module>