Cache maps for externalized typed array objects.

After this, the maps do not garbage-collect, and typed array
constructors do not deopt.

R=ulan@chromium.org
BUG=363855
LOG=N

Review URL: https://codereview.chromium.org/247343002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20887 ce2b1a6d-e550-0410-aec6-3dcde31c8c00