Remove the LookupResult IsValid method because it is confusing.

Remove the LookupResult IsValid method because it is confusing.

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