LibWeb: Cache the viewport rect across all of style computation

Fetching the viewport rect is currently somewhat expensive, since it
requires finding the navigable the document is active in.

We can avoid the cost of repeated calls by simply allowing StyleComputer
to cache the viewport rect at the start of style computation.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1129,6 +1129,9 @@ void Document::update_style()
     if (m_created_for_appropriate_template_contents)
         return;
 
+    // Fetch the viewport rect once, instead of repeatedly, during style computation.
+    style_computer().set_viewport_rect({}, viewport_rect());
+
     evaluate_media_rules();
 
     auto invalidation = update_style_recursively(*this);