Fix Heap::IsHeapIterable.

We only consider heap iterable if the new space is empty (in addition to the exisiting old space check).

The change also moves the iterability forcing + allocation prevention gadgets to HeapIterator so that it is impossible to miss them when iterating the heap.

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

Review URL: https://codereview.chromium.org/285693006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21387 ce2b1a6d-e550-0410-aec6-3dcde31c8c00