Removed IdleGlobalGC and just perform IdleNotification when incremental marking is on.

BUG=397026
LOG=n
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/422013002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22647 ce2b1a6d-e550-0410-aec6-3dcde31c8c00