impact on the normal idle notification scheme by making sure
we do not restart the idle GC sequence just because of a
forced GC due to context disposal.
Review URL: http://codereview.chromium.org/669150

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4031 ce2b1a6d-e550-0410-aec6-3dcde31c8c00