Revert "Don't force layout for mouse event hit tests"

LayoutTests/fast/events/script-tests/page-scaled-mouse-click-iframe.js

 description("This tests that page scaling and CSS transforms do not affect mouse event pageX and pageY coordinates for " +
             "content embedded in an iframe.");
 
 var html = document.documentElement;
 
 var iframe = document.createElement("iframe");
 iframe.style.border = "none";
 iframe.style.width = "200px";
 iframe.style.height = "200px";
 iframe.style.background = "green";
@@ skipping 57 matching lines @@
     html.style["-webkit-transform-origin"] = "0 0";
     testEvents(0, "CSS scale(0.5, 2.0)", "click(20, 5)");
 
     window.internals.setPageScaleFactor(0.5, 0, 0);
     testEvents(0, "setPageScale(0.5), CSS scale(0.5, 2.0)", "click(40, 10)");
   }
 
   finishJSTest();
 }
 
+// Mouse events only work after an initial layout
+document.body.offsetLeft;
+
 // Add the iframe to the document.
 iframe.src = "resources/page-scaled-mouse-click-iframe-inner.html";
 document.body.insertBefore(iframe, document.body.firstChild);
 
 window.jsTestIsAsync = true;