LibWeb: Don't fire resize event until document actually resizes once

The first time Document learns its viewport size, we now suppress firing
of the resize event.

This fixes an issue on multiple websites that were not expecting resize
events to fire so early in the loading process.

Tests/LibWeb/Text/input/no-window-resize-on-load.html

@@ -0,0 +1,12 @@
+<script>
+    var resizeCount = 0;
+    onresize = function() {
+        ++resizeCount;
+    }
+</script>
+<script src="include.js"></script>
+<script>
+    test(() => {
+        println("resize count: " + resizeCount);
+    })
+</script>