Use a split toggle button to reflect scaling state.

remoting/webapp/me2mom/remoting.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var remoting = remoting || {};
 
 (function() {
 'use strict';
 
 window.addEventListener('blur', pluginLostFocus_, false);
@@ skipping 50 matching lines @@
 var kSupportIdLen = 7;
 var kHostSecretLen = 5;
 var kAccessCodeLen = kSupportIdLen + kHostSecretLen;
 var kDigitsPerGroup = 4;
 
 function hasClass(classes, cls) {
   return classes.match(new RegExp('(\\s|^)' + cls + '(\\s|$)'));
 }
 
 function addClass(element, cls) {
-  if (!hasClass(element.className, cls))
-    element.className = element.className + ' ' + cls;
+  if (!hasClass(element.className, cls)) {
+    var padded = element.className == '' ? '' : element.className + ' ';
+    element.className = padded + cls;
+  }
 }
 
 function removeClass(element, cls) {
   element.className =
-      element.className.replace(new RegExp('\\b' + cls + '\\b', 'g'), '');
+      element.className.replace(new RegExp('\\b' + cls + '\\b', 'g'), '')
+                       .replace('  ', ' ');
 }
 
 function retrieveEmail_(access_token) {
   var headers = {
     'Authorization': 'OAuth ' + remoting.oauth2.getAccessToken()
   };
 
   var onResponse = function(xhr) {
     if (xhr.status != 200) {
       // TODO(ajwong): Have a better way of showing an error.
@@ skipping 48 matching lines @@
   var debugLog = document.getElementById('debug-log');
   if (debugLog.hidden) {
     debugLog.hidden = false;
   } else {
     debugLog.hidden = true;
   }
 }
 
 remoting.init = function() {
   l10n.localize();
+  var button = document.getElementById('toggle-scaling');
+  button.title = chrome.i18n.getMessage(/*i18n-content*/'TOOLTIP_SCALING');
   // Create global objects.
   remoting.oauth2 = new remoting.OAuth2();
   remoting.debug =
       new remoting.DebugLog(document.getElementById('debug-messages'));
 
   refreshEmail_();
   var email = getEmail();
   if (email) {
     document.getElementById('current-email').innerText = email;
   }
@@ skipping 434 matching lines @@
  * @return {boolean} True if Host mode is supported.
  */
 function isHostModeSupported() {
   // Currently, ChromeOS is not supported.
   return !navigator.userAgent.match(/\bCrOS\b/);
 }
 
 /**
  * Enable or disable scale-to-fit.
  *
+ * @param {boolean} scale True to enable scaling.
  * @return {void} Nothing.
  */
-remoting.toggleScaleToFit = function() {
+remoting.toggleScaleToFit = function(button) {
   remoting.scaleToFit = !remoting.scaleToFit;
-  remoting.session.toggleScaleToFit(remoting.scaleToFit);
+  if (remoting.scaleToFit) {
+    addClass(button, 'toggle-button-active');
+  } else {
+    removeClass(button, 'toggle-button-active');
+  }
+  remoting.session.setScaleToFit(remoting.scaleToFit);
 }
 
 /**
  * Disconnect the remoting client.
  *
  * @return {void} Nothing.
  */
 remoting.disconnect = function() {
   if (remoting.session) {
     remoting.session.disconnect();
@@ skipping 18 matching lines @@
       var result = chrome.i18n.getMessage(/*i18n-content*/'CLOSE_PROMPT');
       return result;
     default:
       return NULL;
   }
 }
 
 }());
 
 // Shortcut to save typing now that this is the only way to show the debug log.
-var tdl = remoting.toggleDebugLog;
+var tdl = remoting.toggleDebugLog;