WebUI: restrict cr-dialogs max-height to the viewport.

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() {
   function pressEnter(element) {
     MockInteractions.keyEventOn(element, 'keypress', 13, undefined, 'Enter');
   }
 
   setup(function() {
@@ skipping 154 matching lines @@
       assertFalse(bodyContainer.classList.contains('top-scrollable'));
       assertFalse(bodyContainer.classList.contains('bottom-scrollable'));
       done();
     }, 1);
   });
 
   test('dialog body scrollable border when appropriate', function(done) {
     document.body.innerHTML = `
       <dialog is="cr-dialog" show-scroll-borders>
         <div class="title">title</div>
-        <div class="body">body</div>
+        <div class="body">
+          <div style="height: 100px">tall content</div>
+        </div>
       </dialog>`;
 
     var dialog = document.body.querySelector('dialog');
     var bodyContainer = dialog.$$('.body-container');
     assertTrue(!!bodyContainer);
 
     dialog.showModal();  // Attach the dialog for the first time here.
 
     var observerCount = 0;
 
@@ skipping 20 matching lines @@
           assertTrue(bodyContainer.classList.contains('bottom-scrollable'));
           assertTrue(bodyContainer.classList.contains('top-scrollable'));
           observer.disconnect();
           done();
           break;
       }
     });
     observer.observe(bodyContainer, {attributes: true});
 
     // Height is normally set via CSS, but mixin doesn't work with innerHTML.
-    bodyContainer.style.height = '1px';
+    bodyContainer.style.height = '60px';  // Element has "min-height: 60px".
     bodyContainer.scrollTop = 100;
   });
 });