Refactor JavaScriptFrame::function() to return a JSFunction* and remove associated casts.

BUG=

Review URL: https://codereview.chromium.org/18404009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15638 ce2b1a6d-e550-0410-aec6-3dcde31c8c00