Use atomic operation to read the length of a fixed array.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index f2ca0800adb96ccfd52820066a77ee4dac699902..78a77e233e54ec5cf0928ee7a6635e51420d7432 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -3497,7 +3497,7 @@ bool Heap::CanMoveObjectStart(HeapObject* object) {
 void Heap::AdjustLiveBytes(Address address, int by, InvocationMode mode) {
   if (incremental_marking()->IsMarking() &&
       Marking::IsBlack(Marking::MarkBitFrom(address))) {
-    if (mode == FROM_GC) {
+    if (mode == SWEEPING_IS_OFF) {
       MemoryChunk::IncrementLiveBytesFromGC(address, by);
     } else {
       MemoryChunk::IncrementLiveBytesFromMutator(address, by);
@@ -3546,7 +3546,7 @@ FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
 
   // Maintain consistency of live bytes during incremental marking
   marking()->TransferMark(object->address(), new_start);
-  AdjustLiveBytes(new_start, -bytes_to_trim, Heap::FROM_MUTATOR);
+  AdjustLiveBytes(new_start, -bytes_to_trim, Heap::SWEEPING_STARTED);
 
   // Notify the heap profiler of change in object layout.
   OnMoveEvent(new_object, object, new_object->Size());
@@ -3555,10 +3555,10 @@ FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
 
 
 // Force instantiation of templatized method.
-template
-void Heap::RightTrimFixedArray<Heap::FROM_GC>(FixedArrayBase*, int);
-template
-void Heap::RightTrimFixedArray<Heap::FROM_MUTATOR>(FixedArrayBase*, int);
+template void Heap::RightTrimFixedArray<Heap::SWEEPING_IS_OFF>(FixedArrayBase*,
+                                                               int);
+template void Heap::RightTrimFixedArray<Heap::SWEEPING_STARTED>(FixedArrayBase*,
+                                                                int);
 
 
 template<Heap::InvocationMode mode>