idle notifications after the one that causes the mark-compact
collection unless four or more garbage collections (scavenges) have
occurred.

The embedder should stop sending idle notifications once V8 returns
true from the IdleNotification call. This change is being defensive so
it will not hurt as badly if embedders continue to send idle
notifications.

Review URL: http://codereview.chromium.org/5726005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5981 ce2b1a6d-e550-0410-aec6-3dcde31c8c00