Consolidate the runtime logic for fetching elements from Objects and JSObjects so that element access in the GetElement cases that are not handled in generated code.

R=svenpanne@chromium.org
BUG=none
TEST=closure.js test faster

Review URL: http://codereview.chromium.org/7569001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8815 ce2b1a6d-e550-0410-aec6-3dcde31c8c00