This CL is an intermediate step only, in the end we need to have a single
DefineOrRedefineAccessorProperty call for a single Object.defineProperty
call. Currently we can end up making two such calls, making the necessary access
checks extremely ugly and hard (impossible?) to get right for complete spec
conformance.

The bulk of the change is quite mechanical:

 * Prepare an AccessorPair *before* we add it to our data structures,
   eliminating the previous voodoo-like threading of a placeholder.

 * The previous item makes it possible to activate our check that we do not
   share AccessorPairs by accident.

 * Split a monster method into 2 quite unrelated methods.

 * Use templated To method in a few places.

Review URL: https://chromiumcodereview.appspot.com/9428026

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10788 ce2b1a6d-e550-0410-aec6-3dcde31c8c00