Fix modal-dialog-scroll-height.html

This test is failing in all browsers on wpt.fyi due to the hard coded
height:
https://wpt.fyi/results/html/semantics/interactive-elements/the-dialog-element/modal-dialog-scroll-height.html

I'm not sure why it passes in the chromium test infrastructure, but
people want to remove it:
web-platform-tests/interop#12 (comment)

This patch replaces the hard coded height with window.innerHeight.

Change-Id: If27a4b07ca235190d19cbe2f52bbfe6dd2594225

html/semantics/interactive-elements/the-dialog-element/modal-dialog-scroll-height.html

@@ -27,6 +27,6 @@
 <script>
 test(() => {
   document.querySelector('dialog').showModal();
-  assert_equals(document.scrollingElement.scrollHeight, 600);
+  assert_equals(document.scrollingElement.scrollHeight, window.innerHeight);
 }, 'dialogs should be centered before computing overflow.');
 </script>