Handle NaN value storing in Uint8Clamped typed arrays.

TEST=mjsunit/external-array
BUG=
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/667113002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24766 ce2b1a6d-e550-0410-aec6-3dcde31c8c00