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

Committed: http://code.google.com/p/v8/source/detail?r=4476

[Mads Ager (chromium), 2010-04-23 06:48:37]

LGTM

http://codereview.chromium.org/1700005/diff/6001/7004
File src/mark-compact.cc (right):

http://codereview.chromium.org/1700005/diff/6001/7004#newcode1396
src/mark-compact.cc:1396: ASSERT(((first_empty_page == NULL) &&
Now that you have an #ifdef DEBUG here in any case it would be more readable
with ifs and multiple ASSERTS. :)

Another small comment: Kevin made me aware that Page has an is_valid() method
that can be used instead of explicit checking Page pointers against NULL. Also,
it seems that in most other code, we use Page::FromAddress(NULL) instead of
using NULL explicitly in initializations.