Introduce lazy sweeping.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index a4596d38c2c777e6222bef4d488d1831d2f4d018..46f8dc9389ed0780b3d2a98e5945601257a4d4f7 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -478,7 +478,7 @@ bool Heap::CollectGarbage(AllocationSpace space, GarbageCollector collector) {
 
   if (collector == MARK_COMPACTOR &&
       !mark_compact_collector()->PreciseSweepingRequired() &&
-      incremental_marking()->IsMarking() &&
+      incremental_marking()->IsMarkingIncomplete() &&
       !incremental_marking()->should_hurry() &&
       FLAG_incremental_marking_steps) {
     if (FLAG_trace_incremental_marking) {
@@ -1003,6 +1003,9 @@ void Heap::Scavenge() {
 
   incremental_marking()->PrepareForScavenge();
 
+  old_pointer_space()->AdvanceSweeper(new_space_.Size());
+  old_data_space()->AdvanceSweeper(new_space_.Size());
+
   // Flip the semispaces.  After flipping, to space is empty, from space has
   // live objects.
   new_space_.Flip();