Introduce lazy sweeping.

src/incremental-marking-inl.h

Index: src/incremental-marking-inl.h
diff --git a/src/incremental-marking-inl.h b/src/incremental-marking-inl.h
index 33ffce335e7b0895a0ccc77b279d997c341041c9..37b0248928088ded2bdfe074edfea0ac94398293 100644
--- a/src/incremental-marking-inl.h
+++ b/src/incremental-marking-inl.h
@@ -35,7 +35,7 @@ namespace internal {
 
 
 void IncrementalMarking::RecordWrite(HeapObject* obj, Object* value) {
-  if (!IsStopped() && value->IsHeapObject()) {
+  if (IsMarking() && value->IsHeapObject()) {
     MarkBit value_bit = heap_->marking()->MarkBitFrom(HeapObject::cast(value));
     if (IsWhite(value_bit)) {
       MarkBit obj_bit = heap_->marking()->MarkBitFrom(obj);
@@ -49,7 +49,7 @@ void IncrementalMarking::RecordWrite(HeapObject* obj, Object* value) {
 
 
 void IncrementalMarking::RecordWriteOf(HeapObject* value) {
-  if (state_ != STOPPED) {
+  if (IsMarking()) {
     MarkBit value_bit = heap_->marking()->MarkBitFrom(value);
     if (IsWhite(value_bit)) {
       WhiteToGreyAndPush(value, value_bit);
@@ -60,7 +60,7 @@ void IncrementalMarking::RecordWriteOf(HeapObject* value) {
 
 
 void IncrementalMarking::RecordWrites(HeapObject* obj) {
-  if (!IsStopped()) {
+  if (IsMarking()) {
     MarkBit obj_bit = heap_->marking()->MarkBitFrom(obj);
     if (IsBlack(obj_bit)) {
       BlackToGreyAndUnshift(obj, obj_bit);