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.
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