Fix Iteration on gc stress builder.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/17069004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15178 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/cctest/test-heap.cc

@@ -877,6 +877,7 @@ TEST(StringAllocation) {
 static int ObjectsFoundInHeap(Heap* heap, Handle<Object> objs[], int size) {
   // Count the number of objects found in the heap.
   int found_count = 0;
+  heap->EnsureHeapIsIterable();
   HeapIterator iterator(heap);
   for (HeapObject* obj = iterator.next(); obj != NULL; obj = iterator.next()) {
     for (int i = 0; i < size; i++) {