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 6ee6c094bd05ebefb228697f0ca778e3b94ff906..2fb9114fc227087c2dc7328f312377634a1c6d3a 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -3452,7 +3452,7 @@ 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 == SEQUENTIAL_TO_SWEEPER) {
       MemoryChunk::IncrementLiveBytesFromGC(address, by);
     } else {
       MemoryChunk::IncrementLiveBytesFromMutator(address, by);
@@ -3502,7 +3502,7 @@ 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::CONCURRENT_TO_SWEEPER);
 
   // Notify the heap profiler of change in object layout.
   OnMoveEvent(new_object, object, new_object->Size());
@@ -3511,10 +3511,10 @@ 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::SEQUENTIAL_TO_SWEEPER>(
+    FixedArrayBase*, int);
+template void Heap::RightTrimFixedArray<Heap::CONCURRENT_TO_SWEEPER>(
+    FixedArrayBase*, int);
 
 
 template<Heap::InvocationMode mode>