TransitionArray::Search() now returns insertion index if the entry was not found.

This is a prerequisite for https://codereview.chromium.org/661133002/.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/698043003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00