Handle CALLBACKS correctly in IsProperty functions.

Handle CALLBACKS correctly in IsProperty functions.

With transitions in AccessorPairs, it is not enough to look at the PropertyType
alone to decide whether we look at a property or not: For objects with
JavaScript accessors, we have to look into the AccessorPair itself and see if
one of its 2 parts is actually a JavaScript accessor. Therefore, a predicate
with a PropertyType argument alone doesn't make sense anymore, we might need the
associated value, too.

Things are complicated by the fact that the holder in a LookupResult can be
NULL, so we must be careful to retrieve its value only when it is really
needed. To achieve the needed call-by-name semantics, a new Entry is introduced,
which is basically a closure over a DescriptorArray and an index into this array
(C++0x to the rescue!). GCC is clever enough to inline this class, so we pay no
runtime penalty for this abstraction.

It's all a bit ugly, but this is caused by the current structure of Descriptor,
DescriptorArray and LookupResult: Things would be much easier if DescriptorArray
were, well, an array of Descriptors, and LookupResult were a 'Maybe Descriptor'
(in Haskell-terms).

Review URL: https://chromiumcodereview.appspot.com/9466047

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10847 ce2b1a6d-e550-0410-aec6-3dcde31c8c00