1) Don't try to sample the stack if VM is in 'GC' state
2) Show GC ticks in profiler statistics

Review URL: http://codereview.chromium.org/27213

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1380 ce2b1a6d-e550-0410-aec6-3dcde31c8c00