Replaced IsValid by IsPropertyOrTransition and used IsProperty in most
of the places where IsValid was used before.  Most of the time when
inspecting a lookup result we really want to know if there is a real
property present.  Only for stores are we interested in transitions.

BUG=http://crbug.com/20104
TEST=cctest/test-api/NamedInterceptorMapTransitionRead
Review URL: http://codereview.chromium.org/647015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3901 ce2b1a6d-e550-0410-aec6-3dcde31c8c00