zero-extended as it should be.  Therefore, the index into the pixel
array could influence the value on reads.

BUG=http://code.google.com/p/chromium/issues/detail?id=26337
Review URL: http://codereview.chromium.org/399067

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3328 ce2b1a6d-e550-0410-aec6-3dcde31c8c00