Wait in heap tests for active concurrent sweeper threads.

BUG=

Review URL: https://codereview.chromium.org/12708003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13895 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/cctest/test-heap.cc

@@ -50,7 +50,11 @@ static void InitializeVM() {
 
 // Go through all incremental marking steps in one swoop.
 static void SimulateIncrementalMarking() {
+  MarkCompactCollector* collector = HEAP->mark_compact_collector();
   IncrementalMarking* marking = HEAP->incremental_marking();
+  if (collector->IsConcurrentSweepingInProgress()) {
+    collector->WaitUntilSweepingCompleted();
+  }
   CHECK(marking->IsMarking() || marking->IsStopped());
   if (marking->IsStopped()) {
     marking->Start();