Object.observe: disable freezing changeRecords until it can be made fast

R=rossberg,adamk
BUG=

Review URL: https://codereview.chromium.org/15017003

Patch from Rafael Weinstein <rafaelw@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14569 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Add Persistent<T>::Reset which disposes the handle and redirects it to point to another object.

BUG=
R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/14788013

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14570 ce2b1a6d-e550-0410-aec6-3dcde31c8c00