Adapt fragmentation heuristics for over reserved pages.

Adapt fragmentation heuristics for over reserved pages.

This adapts the heuristics that detect fragmented pages to reduce memory
footprint for spaces with over reserved memory. This minimizes external
fragmentation caused by pages that cannot be released to the OS because
of just a few live objects on them.

R=erik.corry@gmail.com
TEST=cctest/test-heap/ReleaseOverReservedPages

Review URL: https://chromiumcodereview.appspot.com/10629004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11901 ce2b1a6d-e550-0410-aec6-3dcde31c8c00