Attempt no. 3 to fix Heap::IsHeapIterable and HeapIterator.

Now we remember new space's top pointer after the last GC to find out if there was a new space allocation since the last GC.

Unfortunately, this not completely safe - the debugger has a callback hook (that can call to JS) at the end of the GC epilogue that can in theory allocate and possibly make the heap non-iterable. We can only hope this does not happen.

BUG=373283
R=hpayer@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/291193005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21431 ce2b1a6d-e550-0410-aec6-3dcde31c8c00