Use size_t in GCIdleTimeHandler to fix undefined behaviour.

src/heap/gc-idle-time-handler.h

 // 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.
 
 #ifndef V8_HEAP_GC_IDLE_TIME_HANDLER_H_
 #define V8_HEAP_GC_IDLE_TIME_HANDLER_H_
 
 #include "src/globals.h"
 
 namespace v8 {
 namespace internal {
 
 // The idle time handler makes decisions about which garbage collection
 // operations are executing during IdleNotification.
 class GCIdleTimeHandler {
  public:
-  static intptr_t EstimateMarkingStepSize(
-      int idle_time_in_ms, intptr_t marking_speed_in_bytes_per_ms);
+  static size_t EstimateMarkingStepSize(size_t idle_time_in_ms,
+                                        size_t marking_speed_in_bytes_per_ms);
 
   // If we haven't recorded any incremental marking events yet, we carefully
   // mark with a conservative lower bound for the marking speed.
-  static const intptr_t kInitialConservativeMarkingSpeed = 100 * KB;
+  static const size_t kInitialConservativeMarkingSpeed = 100 * KB;
+
+  // Maximum marking step size returned by EstimateMarkingStepSize.
+  static const size_t kMaximumMarkingStepSize = 700 * MB;
 
   // We have to make sure that we finish the IdleNotification before
   // idle_time_in_ms. Hence, we conservatively prune our workload estimate.
   static const double kConservativeTimeRatio;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(GCIdleTimeHandler);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_GC_IDLE_TIME_HANDLER_H_