Fix incremental marking speed accelleration and start incremental marking when old space is almost full when pretenuring objects.

BUG=

Review URL: https://codereview.chromium.org/14714004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14559 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Added an extra flag that enables only ArrayBuffer.

This makes Blink experimentation easier.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/14884012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14560 ce2b1a6d-e550-0410-aec6-3dcde31c8c00