Start incremental marking in idle time handler only if it is worthwhile.

src/heap/heap.h

 // 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.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 
 #include "src/allocation.h"
@@ skipping 1914 matching lines @@
   unsigned int maximum_size_scavenges_;
 
   // TODO(hpayer): Allocation site pretenuring may make this method obsolete.
   // Re-visit incremental marking heuristics.
   bool IsHighSurvivalRate() { return high_survival_rate_period_length_ > 0; }
 
   void SelectScavengingVisitorsTable();
 
   void AdvanceIdleIncrementalMarking(intptr_t step_size);
 
+  bool WorthActivatingIncrementalMarking();
+
   void ClearObjectStats(bool clear_last_time_stats = false);
 
   void set_weak_object_to_code_table(Object* value) {
     DCHECK(!InNewSpace(value));
     weak_object_to_code_table_ = value;
   }
 
   Object** weak_object_to_code_table_address() {
     return &weak_object_to_code_table_;
   }
@@ skipping 549 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_