Fix for an assertion failure in Map::FindTransitionToField(...). Appeared after r25136.

BUG=chromium:430846
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/704183002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25185 ce2b1a6d-e550-0410-aec6-3dcde31c8c00