LibWeb: Make LayoutState use HashMap instead of potentially huge Vector

Before this change, LayoutState essentially had a Vector<UsedValues*>
resized to the exact number of layout nodes in the current document.

When a nested layout is performed (to calculate the intrinsic size of
something), we make a new LayoutState with its own Vector. If an entry
is missing in a nested LayoutState, we check the parent chain all the
way up to the root.

Because each nested LayoutState had to allocate a new Vector with space
for all layout nodes, this could get really nasty on very large pages
(such as the ECMA262 specification).

This patch replaces the Vector with a HashMap. There's now a small cost
to lookups, but what we get in return is the ability to handle huge
layout trees without spending eternity in page faults.

Userland/Libraries/LibWeb/Layout/Node.h

@@ -42,8 +42,6 @@ class Node
 public:
     virtual ~Node();
 
-    size_t serial_id() const { return m_serial_id; }
-
     bool is_anonymous() const;
     DOM::Node const* dom_node() const;
     DOM::Node* dom_node();
@@ -165,8 +163,6 @@ private:
 
     JS::NonnullGCPtr<HTML::BrowsingContext> m_browsing_context;
 
-    size_t m_serial_id { 0 };
-
     bool m_anonymous { false };
     bool m_has_style { false };
     bool m_visible { true };