Changed the PageIterator class so that it only returns pages existing

Changed the PageIterator class so that it only returns pages existing
at construction time.  If allocation during iteration causes a paged
space to expand, the iterator will not return the new pages.

This makes it more closely match the HeapObjectIterator behavior, and
it removes a possible source of bugs (if the allocation top was in the
last page in the space, the old iterator would stop only when it
reached the end of the space, potentially returning invalid pages from
a freshly expanded space).

Review URL: http://codereview.chromium.org/115074

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1895 ce2b1a6d-e550-0410-aec6-3dcde31c8c00