[NTP] Disable Done button when textfield is empty

Disable Done button on custom link edit dialog when the url is empty or
spaces only.

Screencast:
https://screencast.googleplex.com/cast/NTk2NTU5NzE1ODQwODE5Mnw3OWZhOGVmNi04Mg

Bug: 886665
Change-Id: Ic674894e73ff8dc183af740505fdaa906e5241d9
Reviewed-on: https://chromium-review.googlesource.com/1243678
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594092}

chrome/browser/resources/local_ntp/custom_links_edit.js

@@ -120,19 +120,6 @@ function prepopulateFields(rid) {
 }
 
 
-/**
- * Disables the "Done" button until the URL field is modified.
- */
-function disableSubmitUntilTextInput() {
-  $(IDS.DONE).disabled = true;
-  let reenable = (event) => {
-    $(IDS.DONE).disabled = false;
-    $(IDS.URL_FIELD).removeEventListener('input', reenable);
-  };
-  $(IDS.URL_FIELD).addEventListener('input', reenable);
-}
-
-
 /**
  * Shows the invalid URL error message until the URL field is modified.
  */
@@ -243,7 +230,7 @@ function handlePostMessage(event) {
       document.title = addLinkTitle;
       $(IDS.DIALOG_TITLE).textContent = addLinkTitle;
       $(IDS.DELETE).disabled = true;
-      disableSubmitUntilTextInput();
+      $(IDS.DONE).disabled = true;
       // Set accessibility names.
       $(IDS.DONE).setAttribute('aria-label', addLinkTitle);
       $(IDS.DONE).title = addLinkTitle;
@@ -352,6 +339,9 @@ function init() {
       .addEventListener('focusin', () => changeColor(IDS.URL_FIELD_NAME));
   $(IDS.URL_FIELD)
       .addEventListener('blur', () => changeColor(IDS.URL_FIELD_NAME));
+  // Disables the "Done" button when the URL field is empty.
+  $(IDS.URL_FIELD).addEventListener('input',
+      () => $(IDS.DONE).disabled = ($(IDS.URL_FIELD).value.trim() === ''));
 
   $(IDS.EDIT_DIALOG).showModal();