Estimate a (close) upper bound on the size of black-marked objects on each page.

src/incremental-marking.h

Index: src/incremental-marking.h
diff --git a/src/incremental-marking.h b/src/incremental-marking.h
index 55fe7e7eba532acc0d48813eacc32c1a3639dc3e..20e28876149e9541e982e8081485c6c3d5a8deae 100644
--- a/src/incremental-marking.h
+++ b/src/incremental-marking.h
@@ -121,12 +121,13 @@ class IncrementalMarking {
   inline void WhiteToGrey(HeapObject* obj, MarkBit mark_bit);
 
   // Does white->black or grey->grey
-  inline void MarkBlackOrKeepGrey(MarkBit mark_bit) {
+  inline bool MarkBlackOrKeepGrey(MarkBit mark_bit) {
     ASSERT(!Marking::IsImpossible(mark_bit));
-    if (mark_bit.Get()) return;
+    if (mark_bit.Get()) return false;
     mark_bit.Set();
     ASSERT(!Marking::IsWhite(mark_bit));
     ASSERT(!Marking::IsImpossible(mark_bit));
+    return true;
   }
 
   inline int steps_count() {