This is per resolution in the WebGL working group to make these types
match the WebIDL IndexSetter and IndexGetter semantics, which are
still being defined but will very likely not throw.

Review URL: http://codereview.chromium.org/328007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3117 ce2b1a6d-e550-0410-aec6-3dcde31c8c00