Provide more accurate results about used heap size via GetHeapStatistics.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 7444c5bf3c27d2f36266ef99e9ec7d052d7a3f1c..1b7e60022e57674093ab5b87a1e53da542759b9a 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -119,6 +119,9 @@ class MarkCompactCollector: public AllStatic {
   // Determine type of object and emit deletion log event.
   static void ReportDeleteIfNeeded(HeapObject* obj);
 
+  // Returns size of a possibly marked object.
+  static int SizeOfMarkedObject(HeapObject* obj);
+
   // Distinguishable invalid map encodings (for single word and multiple words)
   // that indicate free regions.
   static const uint32_t kSingleFreeEncoding = 0;