LibWeb: Don't force relayout whole page on programmatic scroll

When scrolling the page, we may need to flush any pending layout
changes. This is required because otherwise, we don't know if the target
scroll position is valid.

However, we don't need to *force* a layout. If the layout tree is
already up-to-date, we can use it as-is.

Also, if we're scrolling to (0, 0), we don't need to update the layout
at all, since (0, 0) is always a guaranteed valid scroll position.

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -389,8 +389,12 @@ void BrowsingContext::set_needs_display(CSSPixelRect const& rect)
 
 void BrowsingContext::scroll_to(CSSPixelPoint position)
 {
-    if (active_document())
-        active_document()->force_layout();
+    // NOTE: Scrolling to a position requires up-to-date layout *unless* we're scrolling to (0, 0)
+    //       as (0, 0) is always guaranteed to be a valid scroll position.
+    if (!position.is_zero()) {
+        if (active_document())
+            active_document()->update_layout();
+    }
 
     if (m_page)
         m_page->client().page_did_request_scroll_to(position);
@@ -416,7 +420,7 @@ void BrowsingContext::scroll_to_anchor(DeprecatedString const& fragment)
     if (!element)
         return;
 
-    document->force_layout();
+    document->update_layout();
 
     if (!element->layout_node())
         return;