[heap] Do not clear mark bits of left trimmed old object start.

src/heap/mark-compact-inl.h

 // Copyright 2012 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_HEAP_MARK_COMPACT_INL_H_
 #define V8_HEAP_MARK_COMPACT_INL_H_
 
 #include "src/heap/mark-compact.h"
 #include "src/heap/remembered-set.h"
 #include "src/isolate.h"
@@ skipping 161 matching lines @@
         if (T == kBlackObjects || T == kAllLiveObjects) {
           object = black_object;
         }
       } else if ((T == kGreyObjects || T == kAllLiveObjects)) {
         map = base::NoBarrierAtomicValue<Map*>::FromAddress(addr)->Value();
         object = HeapObject::FromAddress(addr);
       }
 
       // We found a live object.
       if (object != nullptr) {
-        if (map == heap()->one_pointer_filler_map()) {
-          // Black areas together with slack tracking may result in black one
-          // word filler objects. We filter these objects out in the iterator.
+        if (object->IsFiller()) {
+          // There are two reasons why we can get black or grey fillers:
+          // 1) Black areas together with slack tracking may result in black one
+          // word filler objects.
+          // 2) Left trimming may leave black or grey fillers behind because we
+          // do not clear the old location of the object start.
+          // We filter these objects out in the iterator.
           object = nullptr;
         } else {
           break;
         }
       }
     }
 
     if (current_cell_ == 0) {
       if (!it_.Done()) {
         it_.Advance();
         cell_base_ = it_.CurrentCellBase();
         current_cell_ = *it_.CurrentCell();
       }
     }
     if (object != nullptr) return object;
   }
   return nullptr;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_MARK_COMPACT_INL_H_