Use size_t in GCIdleTimeHandler to fix undefined behaviour.

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/v8.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/once.h"
 #include "src/base/utils/random-number-generator.h"
@@ skipping 4247 matching lines @@
     CollectAllGarbage(kMakeHeapIterableMask, "Heap::MakeHeapIterable");
   }
   if (mark_compact_collector()->sweeping_in_progress()) {
     mark_compact_collector()->EnsureSweepingCompleted();
   }
   DCHECK(IsHeapIterable());
 }
 
 
 void Heap::AdvanceIdleIncrementalMarking(int idle_time_in_ms) {
-  intptr_t step_size = GCIdleTimeHandler::EstimateMarkingStepSize(
-      idle_time_in_ms, tracer_.IncrementalMarkingSpeedInBytesPerMillisecond());
+  intptr_t step_size =
+      static_cast<size_t>(GCIdleTimeHandler::EstimateMarkingStepSize(
+          idle_time_in_ms,
+          tracer_.IncrementalMarkingSpeedInBytesPerMillisecond()));
 
   incremental_marking()->Step(step_size,
                               IncrementalMarking::NO_GC_VIA_STACK_GUARD, true);
 
   if (incremental_marking()->IsComplete()) {
     bool uncommit = false;
     if (gc_count_at_last_idle_gc_ == gc_count_) {
       // No GC since the last full GC, the mutator is probably not active.
       isolate_->compilation_cache()->Clear();
       uncommit = true;
@@ skipping 1861 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 }
 }  // namespace v8::internal