[heap] Fix clearing slots for map and size during LeftTrimFixedArray

BUG=chromium:598319
LOG=N

Review-Url: https://codereview.chromium.org/1943263002
Cr-Commit-Position: refs/heads/master@{#35970}

src/heap/heap.cc

@@ -3143,9 +3143,9 @@ FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
       FixedArrayBase::cast(HeapObject::FromAddress(new_start));
 
   // Remove recorded slots for the new map and length offset.
-  ClearRecordedSlot(new_object, HeapObject::RawField(object, 0));
-  ClearRecordedSlot(
-      new_object, HeapObject::RawField(object, FixedArrayBase::kLengthOffset));
+  ClearRecordedSlot(new_object, HeapObject::RawField(new_object, 0));
+  ClearRecordedSlot(new_object, HeapObject::RawField(
+                                    new_object, FixedArrayBase::kLengthOffset));
 
   // Maintain consistency of live bytes during incremental marking
   Marking::TransferMark(this, object->address(), new_start);