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/expected/no-window-resize-on-load.txt

@@ -0,0 +1 @@
+resize count: 0

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>

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -2553,11 +2553,14 @@ void Document::run_the_resize_steps()
     //    fire an event named resize at the Window object associated with doc.
 
     auto viewport_size = viewport_rect().size().to_type<int>();
+    bool is_initial_size = !m_last_viewport_size.has_value();
+
     if (m_last_viewport_size == viewport_size)
         return;
     m_last_viewport_size = viewport_size;
 
-    window()->dispatch_event(DOM::Event::create(realm(), UIEvents::EventNames::resize));
+    if (!is_initial_size)
+        window()->dispatch_event(DOM::Event::create(realm(), UIEvents::EventNames::resize));
 
     schedule_layout_update();
 }

Userland/Libraries/LibWeb/DOM/Document.h

@@ -786,7 +786,7 @@ private:
     bool m_page_showing { false };
 
     // Used by run_the_resize_steps().
-    Gfx::IntSize m_last_viewport_size;
+    Optional<Gfx::IntSize> m_last_viewport_size;
 
     HashTable<ViewportClient*> m_viewport_clients;