This adds an additional flag to control whether incremental marking
should be aborted when requesting a GC, providing a finer granularity
between kNoGCFlags and kMakeHeapIterableMask.

R=ulan@chromium.org
BUG=v8:1608

Review URL: https://chromiumcodereview.appspot.com/9608006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10961 ce2b1a6d-e550-0410-aec6-3dcde31c8c00