The heap snapshot generator uses the UnreachableObjectsFilter which in
turn messes with marking bits. This requires incremental marking to be
turned off while the snapshot is being generated.

R=verwaest@chromium.org
TEST=cctest/test-heap-profiler --stress-compaction

Review URL: https://codereview.chromium.org/11413183

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13066 ce2b1a6d-e550-0410-aec6-3dcde31c8c00