Fix identity hash code function to respect flag.

Fix identity hash code function to respect flag.

The flag passed to JSObject::GetIdentityHash() was not respected so far
and an indentity hash code was generated even when the flag requested
not to do so. This could lead to a rare corner cases (for which a test
case was added) where a GC request would have been dropped.

R=rossberg@chromium.org
TEST=cctest/test-dictionary

Review URL: http://codereview.chromium.org/8390047

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9801 ce2b1a6d-e550-0410-aec6-3dcde31c8c00