Let the second pass phantom callbacks run in a separate task on the foreground thread.

src/list.h

Index: src/list.h
diff --git a/src/list.h b/src/list.h
index b636449c423b9d9ea7a184de78a3580a49c5780e..8021a9fbed1e5d955cbf910107f1c26d948ab5f6 100644
--- a/src/list.h
+++ b/src/list.h
@@ -5,6 +5,8 @@
 #ifndef V8_LIST_H_
 #define V8_LIST_H_
 
+#include <algorithm>
+
 #include "src/checks.h"
 #include "src/utils.h"
 
@@ -137,6 +139,9 @@ class List {
   // Drop the last 'count' elements from the list.
   INLINE(void RewindBy(int count)) { Rewind(length_ - count); }
 
+  // Swaps the contents of the two lists.
+  INLINE(void Swap(List<T, AllocationPolicy>* list));
+
   // Halve the capacity if fill level is less than a quarter.
   INLINE(void Trim(AllocationPolicy allocator = AllocationPolicy()));