[gn] Enable stricter build flags

src/runtime/runtime-debug.cc

 // Copyright 2014 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/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/compiler.h"
 #include "src/debug/debug-coverage.h"
 #include "src/debug/debug-evaluate.h"
@@ skipping 1294 matching lines @@
   CONVERT_NUMBER_CHECKED(int32_t, max_references, Int32, args[2]);
   CHECK(max_references >= 0);
 
   List<Handle<JSObject> > instances;
   Heap* heap = isolate->heap();
   {
     HeapIterator iterator(heap, HeapIterator::kFilterUnreachable);
     // Get the constructor function for context extension and arguments array.
     Object* arguments_fun = isolate->sloppy_arguments_map()->GetConstructor();
     HeapObject* heap_obj;
-    while ((heap_obj = iterator.next())) {
+    while ((heap_obj = iterator.next()) != nullptr) {
       if (!heap_obj->IsJSObject()) continue;
       JSObject* obj = JSObject::cast(heap_obj);
       if (obj->IsJSContextExtensionObject()) continue;
       if (obj->map()->GetConstructor() == arguments_fun) continue;
       if (!obj->ReferencesObject(*target)) continue;
       // Check filter if supplied. This is normally used to avoid
       // references from mirror objects.
       if (!filter->IsUndefined(isolate) &&
           HasInPrototypeChainIgnoringProxies(isolate, obj, *filter)) {
         continue;
@@ skipping 32 matching lines @@
   DCHECK_EQ(2, args.length());
   CONVERT_ARG_HANDLE_CHECKED(JSFunction, constructor, 0);
   CONVERT_NUMBER_CHECKED(int32_t, max_references, Int32, args[1]);
   CHECK(max_references >= 0);
 
   List<Handle<JSObject> > instances;
   Heap* heap = isolate->heap();
   {
     HeapIterator iterator(heap, HeapIterator::kFilterUnreachable);
     HeapObject* heap_obj;
-    while ((heap_obj = iterator.next())) {
+    while ((heap_obj = iterator.next()) != nullptr) {
       if (!heap_obj->IsJSObject()) continue;
       JSObject* obj = JSObject::cast(heap_obj);
       if (obj->map()->GetConstructor() != *constructor) continue;
       instances.Add(Handle<JSObject>(obj));
       if (instances.length() == max_references) break;
     }
     // Iterate the rest of the heap to satisfy HeapIterator constraints.
     while (iterator.next()) {
     }
   }
@@ skipping 565 matching lines @@
 
 RUNTIME_FUNCTION(Runtime_DebugTogglePreciseCoverage) {
   SealHandleScope shs(isolate);
   CONVERT_BOOLEAN_ARG_CHECKED(enable, 0);
   Coverage::TogglePrecise(isolate, enable);
   return isolate->heap()->undefined_value();
 }
 
 }  // namespace internal
 }  // namespace v8