Extract "fullscreen element ready check" from requestFullscreen()

LayoutTests/fullscreen/trusted-event.js

Index: LayoutTests/fullscreen/trusted-event.js
diff --git a/LayoutTests/fullscreen/trusted-event.js b/LayoutTests/fullscreen/trusted-event.js
index 1f6a835195ab16310a1027c70e2522dc1a2e9939..9f74186b64c6ccf225c219f7920fd17c3abc9f4c 100644
--- a/LayoutTests/fullscreen/trusted-event.js
+++ b/LayoutTests/fullscreen/trusted-event.js
@@ -17,6 +17,7 @@ function trusted_event(callback, container)
         // 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()
@@ -30,8 +31,10 @@ function trusted_event(callback, container)
 }
 
 // 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);
 }