Keep PersistentValueStorage page size in sync with system page size

Keep PersistentValueStorage page size in sync with system page size

WTF::PageAllocation::allocate() requires the allocated size to be a
multiple of the system page size (available through WTF::pageSize()),
so we can't hard-code the page size in PersistentValueStorage to 4K.

As we control the memory of the page we can use 'Value values[1]' as
a trick to let the compiler set up a head-reference for our list of
Values, which we access by values[0:kEntriesPerPage - 1].

Change-Id: I71fdfffa0424d510fc1073bb67b6675a04ac2d80
Reviewed-by: Lars Knoll <lars.knoll@digia.com>