The flag is only turned on when --trace_gc is one. It prints out used and available bytes in each space. To enable it, ENABLE_LOGGING_AND_PROFILING must be defined.

This is a mini version of --heap_stats, but don't need DEBUG macro to be turned on.

Review URL: http://codereview.chromium.org/149568

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2449 ce2b1a6d-e550-0410-aec6-3dcde31c8c00