This implements incremental scanning of large objects using a progress
bar in the page header of such objects. Note that this requires forward
white to gray transitions in the write barrier and hence is disabled by
default for now.

R=ulan@chromium.org,hpayer@chromium.org

Review URL: https://codereview.chromium.org/11362246

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12978 ce2b1a6d-e550-0410-aec6-3dcde31c8c00