LibHTML: Fix LayoutDocument height computation

Author: https://github.com/bugaevc
Commit: https://github.com/SerenityOS/serenity/commit/03cca4510a7
Pull-request: https://github.com/SerenityOS/serenity/pull/609
Reviewed-by: https://github.com/awesomekling ✅

Libraries/LibHTML/Layout/LayoutDocument.cpp

@@ -12,5 +12,13 @@ LayoutDocument::~LayoutDocument()
 void LayoutDocument::layout()
 {
     rect().set_width(style().size().width());
+
     LayoutNode::layout();
+
+    int lowest_bottom = 0;
+    for_each_child([&](auto& child) {
+        if (child.rect().bottom() > lowest_bottom)
+            lowest_bottom = child.rect().bottom();
+    });
+    rect().set_bottom(lowest_bottom);
 }