Fix test: manually call GC since 0 allocations will not cause GC

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/587663003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24091 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/cctest/test-heap.cc

@@ -2861,6 +2861,7 @@ TEST(TransitionArrayShrinksDuringAllocToOnePropertyFound) {
 
   root = GetByName("root");
   AddPropertyTo(0, root, "prop9");
+  CcTest::i_isolate()->heap()->CollectGarbage(OLD_POINTER_SPACE);
 
   // Count number of live transitions after marking.  Note that one transition
   // is left, because 'o' still holds an instance of one transition target.