Ensure CopyElementsImpl is always executed so it fills in holes even if from_size is 0. Allow FixedDoubleArray::cast to also support FixedArray with size 0.

Review URL: https://chromiumcodereview.appspot.com/11280054

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13000 ce2b1a6d-e550-0410-aec6-3dcde31c8c00