[gn] Enable stricter build flags

src/heap/heap.cc

 // 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.
 
 #include "src/heap/heap.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/assembler-inl.h"
 #include "src/ast/context-slot-cache.h"
@@ skipping 740 matching lines @@
   {
     TRACE_GC(tracer(), GCTracer::Scope::HEAP_EPILOGUE_REDUCE_NEW_SPACE);
     ReduceNewSpaceSize();
   }
 }
 
 
 void Heap::PreprocessStackTraces() {
   WeakFixedArray::Iterator iterator(weak_stack_trace_list());
   FixedArray* elements;
-  while ((elements = iterator.Next<FixedArray>())) {
+  while ((elements = iterator.Next<FixedArray>()) != nullptr) {
     for (int j = 1; j < elements->length(); j += 4) {
       Object* maybe_code = elements->get(j + 2);
       // If GC happens while adding a stack trace to the weak fixed array,
       // which has been copied into a larger backing store, we may run into
       // a stack trace that has already been preprocessed. Guard against this.
       if (!maybe_code->IsAbstractCode()) break;
       AbstractCode* abstract_code = AbstractCode::cast(maybe_code);
       int offset = Smi::cast(elements->get(j + 3))->value();
       int pos = abstract_code->SourcePosition(offset);
       elements->set(j + 2, Smi::FromInt(pos));
@@ skipping 5599 matching lines @@
 }
 
 
 // static
 int Heap::GetStaticVisitorIdForMap(Map* map) {
   return StaticVisitorBase::GetVisitorId(map);
 }
 
 }  // namespace internal
 }  // namespace v8