MD Settings: Display error messages in "add" and "edit" exception dialogs.

chrome/browser/resources/settings/site_settings/add_site_dialog.js

 // Copyright 2016 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.
 
 /**
  * @fileoverview
  * 'add-site-dialog' provides a dialog to add exceptions for a given Content
  * Settings category.
  */
 Polymer({
@@ skipping 37 matching lines @@
         this.onIncognitoStatusChanged_.bind(this));
     this.browserProxy.updateIncognitoStatus();
     this.$.dialog.showModal();
   },
 
   /**
    * Validates that the pattern entered is valid.
    * @private
    */
   validate_: function() {
+    // If input is empty, disable the action button, but don't show the red
+    // invalid message.
+    if (this.$.site.value.trim() == '') {
+      this.$.site.invalid = false;
+      this.$.add.disabled = true;
+      return;
+    }
+
     this.browserProxy.isPatternValid(this.site_).then(function(isValid) {
+      this.$.site.invalid = !isValid;
       this.$.add.disabled = !isValid;
     }.bind(this));
   },
 
   /** @private */
   onCancelTap_: function() {
     this.$.dialog.cancel();
   },
 
   /**
@@ skipping 16 matching lines @@
    */
   onSubmit_: function() {
     if (this.$.add.disabled)
       return;  // Can happen when Enter is pressed.
     this.browserProxy.setCategoryPermissionForOrigin(
         this.site_, this.site_, this.category, this.contentSetting,
         this.$.incognito.checked);
     this.$.dialog.close();
   },
 });