Revert "Track field types."

Revert r20701.

TBR=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/236843002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20704 ce2b1a6d-e550-0410-aec6-3dcde31c8c00