MD Settings: long dialog body should have overscroll line.

chrome/test/data/webui/cr_elements/cr_dialog_test.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.
 
 suite('cr-dialog', function() {
   setup(function() {
     PolymerTest.clearBody();
   });
 
   test('focuses title on show', function() {
@@ skipping 26 matching lines @@
     var button = document.body.querySelector('button');
 
     assertNotEquals(dialog, document.activeElement);
     assertNotEquals(button, document.activeElement);
 
     dialog.showModal();
 
     expectNotEquals(dialog, document.activeElement);
     expectEquals(button, document.activeElement);
   });
+
+  test('dialog body indicates over-scroll when appropriate', function(done) {
+    document.body.innerHTML = `
+      <dialog is="cr-dialog" show-scroll-borders>
+        <div class="title">title</div>
+        <div class="body">body</div>
+      </dialog>`;
+
+    var dialog = document.body.querySelector('dialog');
+    var innerBody = document.body.querySelector('.body');
+    var bodyContainer = dialog.$$('.body-container');
+    assertTrue(!!bodyContainer);
+
+    // Height is normally set via CSS, but mixin doesn't work with innerHTML.
+    bodyContainer.style.height = '1px';
+    innerBody.style.height = '100px';
+    dialog.showModal();
+
+    return PolymerTest.flushTasks().then(function() {

[dpapad, 2017/03/31 01:55:26]

Optional nit: Perhaps this test is more readable if we use the MutationObserver
for both assertions. Similar to as follows

var observerCalls = 0;

var observer = new MutationObserver(function() {
  observerCalls++;
  if (observerCalls == 1) {...assert ....}
  else if (observerCalls == 2) {.... assert....done...}

});
observer.observe(bodyContainer, {attributes: true});

[scottchen, 2017/03/31 18:10:56]

Done.

+      assertTrue(bodyContainer.classList.contains('bottom-scrollable'));
+
+      var observer = new MutationObserver(function() {
+        assertFalse(bodyContainer.classList.contains('bottom-scrollable'));
+        observer.disconnect();
+        done();
+      });
+      observer.observe(bodyContainer, {attributes: true});
+
+      bodyContainer.scrollTop = 100;
+    });
+  });
 });