Don't force layout for mouse event hit tests

LayoutTests/fast/events/script-tests/mouse-click-events.js

 description("This tests what mouse events we send.");
 
 var div = document.createElement("div");
 div.style.width = "100px";
 div.style.height = "100px";
 div.style.backgroundColor = "blue";
 
 var eventLog = "";
 
 function appendEventLog() {
     if (window.eventSender) {
         eventLog += event.type + "(" + event.button + ") ";
     } else {
         debug(event.type + "(" + event.button + ")");
     }
 }
 
 function clearEventLog() {
     eventLog = "";
 }
 
 div.addEventListener("click", appendEventLog, false);
 div.addEventListener("dblclick", appendEventLog, false);
 div.addEventListener("mousedown", appendEventLog, false);
 div.addEventListener("mouseup", appendEventLog, false);
 document.body.insertBefore(div, document.body.firstChild);
 
-if (window.eventSender)
+if (window.eventSender) {
+    // Mouse events only work after an initial layout
+    document.body.offsetLeft;
     eventSender.mouseMoveTo(10, 10);
+}
 
 function sendEvents(button) {
     if (!window.eventSender) {
         debug("This test requires DumpRenderTree.  Click on the blue rect with different mouse buttons to log.")
         return;
     }
     eventSender.mouseDown(button);
     eventSender.mouseUp(button);
     eventSender.mouseDown(button);
     eventSender.mouseUp(button);
     // could test dragging here too
 }
 
 function testEvents(description, button, expectedString) {
     debug(description);
     sendEvents(button);
     shouldBeEqualToString("eventLog", expectedString);
     clearEventLog();
 }
 
 if (window.eventSender) {
     testEvents("Left Mouse Button", 0, "mousedown(0) mouseup(0) click(0) mousedown(0) mouseup(0) click(0) dblclick(0) ");
     testEvents("Middle Mouse Button", 1, "mousedown(1) mouseup(1) click(1) mousedown(1) mouseup(1) click(1) dblclick(1) ");
     testEvents("Right Mouse Button", 2, "mousedown(2) mouseup(2) mousedown(2) mouseup(2) ");
     testEvents("4th Mouse Button", 3, "mousedown(1) mouseup(1) click(1) mousedown(1) mouseup(1) click(1) dblclick(1) ");
 }