Preserve constant function transition when adding the same function.

Preserve constant function transition when adding the same function.

This should help in cases like:
function Constructor() {
  this.foo = constFunction;
  this.bar = "baz";
}

for (...) {
  o = new Constructor();
  // Constant call IC will work.
  o.foo();
  // Inlined property load will see the same map.
  use(o.bar);
}

This change also fixes a latent bug in custom call IC-s for strings
exposed by string-charcodeat.js.

Review URL: http://codereview.chromium.org/3160006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5254 ce2b1a6d-e550-0410-aec6-3dcde31c8c00