Hide tap dragging a11y control in the absence of a touchpad.

BUG=168909
TEST=manual

Review URL: https://chromiumcodereview.appspot.com/11783062

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176011 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/options/browser_options.html

@@ -639,7 +639,7 @@
           </label>
         </div>
       </div>
-      <div class="option-name">
+      <div class="option-name" id="accessibility-tap-dragging">
         <div class="checkbox">
           <label>
             <input id="accessibility-tap-dragging-check"

chrome/browser/resources/options/browser_options.js

@@ -1197,11 +1197,12 @@ cr.define('options', function() {
     },
 
     /**
-     * Show/hide touchpad settings slider.
+     * Show/hide touchpad-related settings.
      * @private
      */
     showTouchpadControls_: function(show) {
       $('touchpad-settings').hidden = !show;
+      $('accessibility-tap-dragging').hidden = !show;
     },
 
     /**