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

src/global-handles.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_GLOBAL_HANDLES_H_
 #define V8_GLOBAL_HANDLES_H_
 
 #include "include/v8.h"
 #include "include/v8-profiler.h"
 
@@ skipping 280 matching lines @@
   int PostScavengeProcessing(int initial_post_gc_processing_count);
   int PostMarkSweepProcessing(int initial_post_gc_processing_count);
   int DispatchPendingPhantomCallbacks();
   void UpdateListOfNewSpaceNodes();
 
   // Internal node structures.
   class Node;
   class NodeBlock;
   class NodeIterator;
   class PendingPhantomCallback;
+  class PendingPhantomCallbacksSecondPassTask;
 
   Isolate* isolate_;
 
   // Field always containing the number of handles to global objects.
   int number_of_global_handles_;
 
   // List of all allocated node blocks.
   NodeBlock* first_block_;
 
   // List of node blocks with used nodes.
@@ skipping 118 matching lines @@
   List<int> new_space_indices_;
   int singleton_handles_[NUMBER_OF_SINGLETON_HANDLES];
 
   DISALLOW_COPY_AND_ASSIGN(EternalHandles);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_GLOBAL_HANDLES_H_