This improves the heap size estimation by not counting lazy swept pages
as completely allocated but use their live bytes counter instead.

R=vegorov@chromium.org
BUG=v8:1893
TEST=cctest/test-heap/TestSizeOfObjects

Review URL: http://codereview.chromium.org/9173001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10383 ce2b1a6d-e550-0410-aec6-3dcde31c8c00