Do not left-trim arrays when concurrent sweeping is active.

src/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1153 matching lines @@
   // failed.
   // Please note this function does not perform a garbage collection.
   MUST_USE_RESULT inline MaybeObject* AllocateRaw(int size_in_bytes,
                                                   AllocationSpace space,
                                                   AllocationSpace retry_space);
 
   // Initialize a filler object to keep the ability to iterate over the heap
   // when shortening objects.
   void CreateFillerObjectAt(Address addr, int size);
 
+  bool CanMoveObjectStart(HeapObject* object);
+
   enum InvocationMode { FROM_GC, FROM_MUTATOR };
 
   // Maintain marking consistency for IncrementalMarking.
   void AdjustLiveBytes(Address address, int by, InvocationMode mode);
 
   // Makes a new native code object
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed. On success, the pointer to the Code object is stored in the
   // self_reference. This allows generated code to reference its own Code
   // object by containing this pointer.
@@ skipping 1938 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_