Revert "Drastically reduce the number of created strings in d8."

This reverts commit 2fc4424b00296cc06d6f7eb4bf31ec9c3d6be68a.

Even symbols shouldn't be shared across Isolates. The CL has to be reworked to
use a (Isolate,C-string)->v8-symbol mapping. Slowly approaching Chrome... :-/

Review URL: https://codereview.chromium.org/11419067

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00