in old space. Before this change we would compute the fragmentation 
limit before the GC, but that means that we take all the garbage 
into account - and consequently we almost never hit the limit (because 
we have a lot of garbage).

This change changes the policy to compact on the *next* GC if we
determine that we have too much fragmentation after doing a GC.
We'll do a GC if we've wasted more than 1MB and more than 15% of
the old space size.

This can be further improved by computing whether or not to compact
during the marking phase.
Review URL: http://codereview.chromium.org/202008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2841 ce2b1a6d-e550-0410-aec6-3dcde31c8c00