[NTP] send message to iframed doodle when expansion is complete

When the iframe finishes resizing send a postMessage to the doodle
iframe letting it know the expansion has finished.

b/118340003

Change-Id: Ic974ca58f51882d932e8de10c5706990f8f462cd
Reviewed-on: https://chromium-review.googlesource.com/c/1323620Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607729}

chrome/browser/resources/local_ntp/local_ntp.js

@@ -911,9 +911,17 @@ function handlePostMessage(event) {
     let height = args.height || null;
     let duration = args.duration || '0s';
     let iframe = $(IDS.LOGO_DOODLE_IFRAME);
+
+    var transitionCallback = function() {
+      iframe.removeEventListener('webkitTransitionEnd', transitionCallback);
+      iframe.contentWindow.postMessage(
+          {cmd: 'resizeComplete'}, 'https://www.google.com');
+    };
+    iframe.addEventListener('webkitTransitionEnd', transitionCallback, false);
+
+    document.body.style.setProperty('--logo-iframe-resize-duration', duration);
     document.body.style.setProperty('--logo-iframe-height', height);
     document.body.style.setProperty('--logo-iframe-width', width);
-    document.body.style.setProperty('--logo-iframe-resize-duration', duration);
   } else if (cmd === 'startEditLink') {
     $(IDS.CUSTOM_LINKS_EDIT_IFRAME)
         .contentWindow.postMessage({cmd: 'linkData', tid: args.tid}, '*');