[gn] Enable stricter build flags

src/debug/debug.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/debug/debug.h"
 
 #include <memory>
 
 #include "src/api.h"
 #include "src/arguments.h"
@@ skipping 1273 matching lines @@
   DCHECK(shared->is_compiled());
   bool baseline_exists = shared->HasBaselineCode();
 
   {
     // TODO(yangguo): with bytecode, we still walk the heap to find all
     // optimized code for the function to deoptimize. We can probably be
     // smarter here and avoid the heap walk.
     HeapIterator iterator(isolate_->heap());
     HeapObject* obj;
 
-    while ((obj = iterator.next())) {
+    while ((obj = iterator.next()) != nullptr) {
       if (obj->IsJSFunction()) {
         JSFunction* function = JSFunction::cast(obj);
         if (!function->Inlines(*shared)) continue;
         if (function->code()->kind() == Code::OPTIMIZED_FUNCTION) {
           Deoptimizer::DeoptimizeFunction(function);
         }
         if (baseline_exists && function->shared() == *shared) {
           functions.Add(handle(function));
         }
       }
@@ skipping 325 matching lines @@
   if (!factory->script_list()->IsWeakFixedArray()) {
     return factory->empty_fixed_array();
   }
   Handle<WeakFixedArray> array =
       Handle<WeakFixedArray>::cast(factory->script_list());
   Handle<FixedArray> results = factory->NewFixedArray(array->Length());
   int length = 0;
   {
     Script::Iterator iterator(isolate_);
     Script* script;
-    while ((script = iterator.Next())) {
+    while ((script = iterator.Next()) != nullptr) {
       if (script->HasValidSource()) results->set(length++, script);
     }
   }
   results->Shrink(length);
   return results;
 }
 
 
 MaybeHandle<Object> Debug::MakeExecutionState() {
   // Create the execution state object.
@@ skipping 805 matching lines @@
     isolate_->Throw(*isolate_->factory()->NewEvalError(
         MessageTemplate::kNoSideEffectDebugEvaluate));
   }
   isolate_->set_needs_side_effect_check(old_needs_side_effect_check_);
   isolate_->debug()->UpdateHookOnFunctionCall();
   isolate_->debug()->side_effect_check_failed_ = false;
 }
 
 }  // namespace internal
 }  // namespace v8