Don't force layout for mouse event hit tests

LayoutTests/fast/events/prevent-drag-to-navigate.html

 <div>PASS</div>
 <script>
 function dragEnter() {
     event.dataTransfer.dropEffect = "none";
     event.preventDefault();
 }
 function dragOver() {
     // See https://bugs.webkit.org/show_bug.cgi?id=25922
     // This is the line which causes failure:
     event.dataTransfer.dropEffect = "none";
@@ skipping 12 matching lines @@
 
 // Capture all drag events
 window.addEventListener("dragenter", dragEnter, true);
 window.addEventListener("dragover", dragOver, true);
 // We don't get a drop event when we prevent drop, so listen for dragleave
 window.addEventListener("dragleave", dragLeave, true);
 
 function doTest() {
     testRunner.dumpAsText();
 
+    // Mouse events only work after an initial layout
+    document.body.offsetLeft;
+
     eventSender.beginDragWithFiles(["resources/file-for-prevent-drag-to-navigate.html"]);
     eventSender.mouseMoveTo(10, 10);
     eventSender.mouseUp();
 }
 
 if (window.eventSender) {
     testRunner.waitUntilDone();
     // The load seems to fail (for the wrong reasons) if we try to kick of a
     // new load before this one is finished.  So we wait.
     window.onload = doTest();
 }
 </script>