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

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

Index: chrome/browser/resources/settings/site_settings/add_site_dialog.js
diff --git a/chrome/browser/resources/settings/site_settings/add_site_dialog.js b/chrome/browser/resources/settings/site_settings/add_site_dialog.js
index 59d1ac63ff905120514d2535913b9842a8a94daa..537f5cb16c0b6b8da9fbf033cee70b910d1fe423 100644
--- a/chrome/browser/resources/settings/site_settings/add_site_dialog.js
+++ b/chrome/browser/resources/settings/site_settings/add_site_dialog.js
@@ -55,7 +55,16 @@ Polymer({
    * @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));
   },