In the ThisNamedPropertyAssignmentFinder, duplicate assignments to the same
property were counted as distinct assignments.  As a simple fix, subsequent
ones overwrite the previously recorded assignment.

This will reorder the assignments, but it is safe since they are restricted
to have only constants and parameters on the right-hand side (and there are
no assignments to the parameters).

R=vegorov@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/8139037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9540 ce2b1a6d-e550-0410-aec6-3dcde31c8c00