Fix bugs introduced by r4475:

Fix bugs introduced by r4475:
- RelinkPageListInChunkOrder might relink unused pages into the middle of a sequence of used pages. Filler objects should be placed at the beginning of such unused pages otherwise generic iterators (e.g. HeapObjectIterator) would not handle them correctly.
- ObjectAreaEnd() should not be used as an allocation limit for pages from FixedSpace. Pages in such spaces do not use top page_extra_ bytes of object area.

TBR=ager@chromium.org
Review URL: http://codereview.chromium.org/1700005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4476 ce2b1a6d-e550-0410-aec6-3dcde31c8c00