This causes map transitions to be treated weakly during incremental
marking and hence allows clearing of non-live transitions. The marking
code is now shared between incremental and non-incremental mode.

R=vegorov@chromium.org
BUG=v8:1465
TEST=cctest/test-heap/Regress1465

Review URL: https://chromiumcodereview.appspot.com/10310168

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11577 ce2b1a6d-e550-0410-aec6-3dcde31c8c00