Don't force layout for mouse event hit tests

LayoutTests/fast/events/mouseup-outside-document.html

 <p>This test verifies that a document that receives a mouse down event receives
 a corresponding mouse up event when the mouse up happens outside the window.
 </p>
 <p>If the test passes, you'll see a PASS message below.
 </p>
 
 <p>To run this test in Safari:
 <ol>
     <li>Mouse down inside the document.</li>
     <li>Move the mouse outside the window.</li>
     <li>Mouse up.</li>
 </ol>
 </p>
 <hr>
 <pre id="log">FAIL: mouse up event did not fire</pre>
 
 <script>
 function main()
 {
     document.addEventListener("mouseup", document_mouseUp, false);
     
     if (window.testRunner)
         testRunner.dumpAsText();
     
     if (window.eventSender) {
+        // Mouse events only work after an initial layout
+        document.body.offsetLeft;
         eventSender.mouseMoveTo(20, 20); // in window
         eventSender.mouseDown();
         eventSender.mouseMoveTo(4000, 215); // outside window
         eventSender.mouseUp();
     }
 }
 
 function document_mouseUp()
 {
     document.getElementById('log').innerHTML = "PASS: mouseup event fired\n";
 }
 
 main();
 </script>