Forward white to grey transition only if slot is before the progress bar of a given object.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 4fbabd6349d43364e9fe387c3ad0352e6ed42462..c246c944c7e819c420031db9a6e7e9c2333d100f 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -503,6 +503,12 @@ class MemoryChunk {
     }
   }
 
+  bool IsLeftOfProgressBar(Object** slot) {
+    Address slot_address = reinterpret_cast<Address>(slot);
+    ASSERT(slot_address > this->address());
+    return (slot_address - this->address() + kObjectStartOffset) <
+           progress_bar();
+  }
 
   static void IncrementLiveBytesFromGC(Address address, int by) {
     MemoryChunk::FromAddress(address)->IncrementLiveBytes(by);