[heap] Minor MC: Implement page moving

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index d3db0a1c2f8fb5fd0c466b237df4a3618037a1e1..5901ff877807e84341f461f73e5dad7000ae3546 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -289,6 +289,10 @@ class MemoryChunk {
 
     // |ANCHOR|: Flag is set if page is an anchor.
     ANCHOR = 1u << 17,
+
+    // |SWEEP_TO_ITERATE|: The page requires sweeping using external markbits
+    // to iterate the page.
+    SWEEP_TO_ITERATE = 1u << 18,
   };
   typedef base::Flags<Flag, uintptr_t> Flags;