Fix JSObject::SetElement for fixed typed array elements.

R=ulan@chromium.org
BUG=357108
LOG=N

Review URL: https://codereview.chromium.org/214543003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20300 ce2b1a6d-e550-0410-aec6-3dcde31c8c00