Don't force layout for mouse event hit tests

LayoutTests/fast/events/clientXY-in-zoom-and-scroll.html

 <script src="../js/resources/js-test-pre.js"></script>
 <style>
     #pusher {
         width: 1000px;
         height: 1000px;
         outline: 1px solid black;
     }
 </style>
 <div id="console"></div>
 <div id="testArea">
@@ skipping 26 matching lines @@
     }
 
     function scrollPage(x, y)
     {
         window.scrollTo(x, y);
     }
 
     if (window.testRunner) {
         testRunner.dumpAsText();
         testRunner.waitUntilDone();
+        // Mouse events only work after an initial layout
+        document.body.offsetLeft;
     }
 
     // Default.
     function base(e)
     {
         event = e;
         debug("Base");
         shouldBe("event.clientX", "100");
         shouldBe("event.clientY", "100");
         shouldBe("event.pageX", "100");
@@ skipping 54 matching lines @@
     window.removeEventListener("click", zoomedAndScrolled, false);
 
     if (window.testRunner) {
         var area = document.getElementById('testArea');
         area.parentNode.removeChild(area);
 
         testRunner.notifyDone();
     }
 </script>
 <script src="../js/resources/js-test-post.js"></script>