Don't force layout for mouse event hit tests

LayoutTests/fast/events/zoom-dblclick.html

 <!-- based on clientXY-in-zoom-and-scroll.html -->
 <script src="../js/resources/js-test-pre.js"></script>
 <script>
     function clickHandler(e)
     {
         // do something
         event = e;
         var elem = document.getElementById('gizmo');
         elem.textContent = "client(" + e.clientX + "," + e.clientY + ") page(" + e.pageX + "," + e.pageY + ")";
     }
@@ skipping 28 matching lines @@
     function zoomPageOut()
     {
         if (window.eventSender) {
             eventSender.zoomPageOut();
         }
     }
 
     if (window.testRunner) {
         testRunner.dumpAsText();
         testRunner.waitUntilDone();
+        // Mouse events only work after an initial layout
+        document.body.offsetLeft;
     }
 
     // Zoomed.
     function zoomed(e)
     {
         event = e;
         debug("\nZoomed");
         shouldBe("event.clientX", "166");
         shouldBe("event.clientY", "83");
         shouldBe("event.pageX", "166");
         shouldBe("event.pageY", "83");
     }
     window.addEventListener("dblclick", zoomed, false);
     zoomPageIn();
     sendDblClick();
     zoomPageOut();
     window.removeEventListener("dblclick", zoomed, 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>