Extract "fullscreen element ready check" from requestFullscreen()

LayoutTests/fullscreen/trusted-event.js

 // Invokes callback from a trusted event.
 // When testing manually, a button is added to the container.
 function trusted_event(callback, container)
 {
     var document = container.ownerDocument;
 
     if (window.testRunner) {
         // Running under LayoutTests. Use timeout to be async.
         setTimeout(function()
         {
             document.addEventListener("click", callback);
             eventSender.mouseDown();
             eventSender.mouseUp();
             document.removeEventListener("click", callback);
         }, 0);
     } else {
         // Running as manual test. Show a button to click.
         var button = document.createElement("button");
         button.textContent = "click to run test";
+        button.style.display = "block";
         button.style.fontSize = "20px";
         button.style.padding = "10px";
         button.onclick = function()
         {
             callback();
             button.onclick = null;
             container.removeChild(button);
         };
         container.appendChild(button);
     }
 }
 
 // Invokes element.requestFullscreen() from a trusted event.
-function trusted_request(element)
+// When testing manually, a button is added to the container,
+// or to element's parent if no container is provided.
+function trusted_request(element, container)
 {
     var request = element.requestFullscreen.bind(element);
-    trusted_event(request, element.parentNode);
+    trusted_event(request, container || element.parentNode);
 }