Ensure that all objects that had the same map before attaching an external array have the same map once the external array is attached.

BUG=75639
TEST=fast/canvas/webgl/uninitialized-test.html

Review URL: http://codereview.chromium.org/6685073

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7318 ce2b1a6d-e550-0410-aec6-3dcde31c8c00