Fix map modification in transition tree traversal.

Fix map modification in transition tree traversal.

While traversing the transition tree we build a work-list using the map
field of maps. Setting those map values with a write barrier causes
black-to-gray changes on maps which are currently not recognized as
such, hence their computed size might be off.

R=vegorov@chromium.org
BUG=v8:1672
TEST=cctest/test-decls/Present

Review URL: http://codereview.chromium.org/8082023

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9498 ce2b1a6d-e550-0410-aec6-3dcde31c8c00