Fix early exit condition for LowMemoryNotification.

When computing next_gc_likely_to_collect_more do not take into account already
free nodes in PostGarbageCollectionProcessing.

This reduces the number of full GC in LowMemoryNotification from 7 to ~2.

R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/18834002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15550 ce2b1a6d-e550-0410-aec6-3dcde31c8c00