In WebKit, we would like to store a void* to a data structure that contains
lots of exciting per-context data. The current API restricts us to storing only
Strings, which is less useful.

I've also cleaned up the implementation of GetData to be less convoluted.

Review URL: https://codereview.chromium.org/10907189
Patch from Adam Barth <abarth@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12520 ce2b1a6d-e550-0410-aec6-3dcde31c8c00